Vehicle brake system

ABSTRACT

A brake system includes first and second wheel brakes, a reservoir, and a brake pedal unit having a housing and a pair of output pistons slidably disposed in the housing. The output pistons generate brake actuating pressure during a manual push-through mode for actuating the first and second wheel brakes. The system further includes a plunger assembly having a housing having a motor driving an actuator, and a piston connected to the actuator. The piston pressurizes a chamber when the piston is moving in a first direction to provide fluid flow out of a first port and through a pair of parallel valves. The piston pressurizes a second chamber when the piston is moving in a second direction opposite the first direction to provide fluid flow out of a second port. The first and second ports are selectively in fluid communication with the wheel brakes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/868,187, filed Sep. 28, 2015, which claims the benefit ofthe earlier filing date of U.S. patent application Ser. No. 13/843,587,filed Mar. 15, 2013, U.S. Provisional Application No. 62/068,134, filedOct. 24, 2014, and U.S. Provisional Application No. 62/055,698, filedSep. 26, 2014, the entirety of each of which is incorporated herein byreference. This application further claims the benefit of U.S.Provisional Application No. 62/592,947, filed Nov. 30, 2017, thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates in general to vehicle braking systems. Vehiclesare commonly slowed and stopped with hydraulic brake systems. Thesesystems vary in complexity but a base brake system typically includes abrake pedal, a tandem master cylinder, fluid conduits arranged in twosimilar but separate brake circuits, and wheel brakes in each circuit.The driver of the vehicle operates a brake pedal which is connected tothe master cylinder. When the brake pedal is depressed, the mastercylinder generates hydraulic forces in both brake circuits bypressurizing brake fluid. The pressurized fluid travels through thefluid conduit in both circuits to actuate brake cylinders at the wheelsto slow the vehicle.

Base brake systems typically use a brake booster which provides a forceto the master cylinder which assists the pedal force created by thedriver. The booster can be vacuum or hydraulically operated. A typicalhydraulic booster senses the movement of the brake pedal and generatespressurized fluid which is introduced into the master cylinder. Thefluid from the booster assists the pedal force acting on the pistons ofthe master cylinder which generate pressurized fluid in the conduit influid communication with the wheel brakes. Thus, the pressures generatedby the master cylinder are increased. Hydraulic boosters are commonlylocated adjacent the master cylinder piston and use a boost valve tocontrol the pressurized fluid applied to the booster.

Braking a vehicle in a controlled manner under adverse conditionsrequires precise application of the brakes by the driver. Under theseconditions, a driver can easily apply excessive braking pressure thuscausing one or more wheels to lock, resulting in excessive slippagebetween the wheel and road surface. Such wheel lock-up conditions canlead to greater stopping distances and possible loss of directionalcontrol.

Advances in braking technology have led to the introduction of Anti-lockBraking Systems (ABS). An ABS system monitors wheel rotational behaviorand selectively applies and relieves brake pressure in the correspondingwheel brakes in order to maintain the wheel speed within a selected sliprange to achieve maximum braking force. While such systems are typicallyadapted to control the braking of each braked wheel of the vehicle, somesystems have been developed for controlling the braking of only aportion of the plurality of braked wheels.

Electronically controlled ABS valves, comprising apply valves and dumpvalves, are located between the master cylinder and the wheel brakes.The ABS valves regulate the pressure between the master cylinder and thewheel brakes. Typically, when activated, these ABS valves operate inthree pressure control modes: pressure apply, pressure dump and pressurehold. The apply valves allow pressurized brake fluid into respectiveones of the wheel brakes to increase pressure during the apply mode, andthe dump valves relieve brake fluid from their associated wheel brakesduring the dump mode. Wheel brake pressure is held constant during thehold mode by closing both the apply valves and the dump valves.

To achieve maximum braking forces while maintaining vehicle stability,it is desirable to achieve optimum slip levels at the wheels of both thefront and rear axles. During vehicle deceleration different brakingforces are required at the front and rear axles to reach the desiredslip levels. Therefore, the brake pressures should be proportionedbetween the front and rear brakes to achieve the highest braking forcesat each axle. ABS systems with such ability, known as Dynamic RearProportioning (DRP) systems, use the ABS valves to separately controlthe braking pressures on the front and rear wheels to dynamicallyachieve optimum braking performance at the front and rear axles underthe then current conditions.

A further development in braking technology has led to the introductionof Traction Control (TC) systems. Typically, valves have been added toexisting ABS systems to provide a brake system which controls wheelspeed during acceleration. Excessive wheel speed during vehicleacceleration leads to wheel slippage and a loss of traction. Anelectronic control system senses this condition and automaticallyapplies braking pressure to the wheel cylinders of the slipping wheel toreduce the slippage and increase the traction available. In order toachieve optimal vehicle acceleration, pressurized brake fluid is madeavailable to the wheel cylinders even if the master cylinder is notactuated by the driver.

During vehicle motion such as cornering, dynamic forces are generatedwhich can reduce vehicle stability. A Vehicle Stability Control (VSC)brake system improves the stability of the vehicle by counteractingthese forces through selective brake actuation. These forces and othervehicle parameters are detected by sensors which signal an electroniccontrol unit. The electronic control unit automatically operatespressure control devices to regulate the amount of hydraulic pressureapplied to specific individual wheel brakes. In order to achieve optimalvehicle stability, braking pressures greater than the master cylinderpressure must quickly be available at all times.

Brake systems may also be used for regenerative braking to recaptureenergy. An electromagnetic force of an electric motor/generator is usedin regenerative braking for providing a portion of the braking torque tothe vehicle to meet the braking needs of the vehicle. A control modulein the brake system communicates with a powertrain control module toprovide coordinated braking during regenerative braking as well asbraking for wheel lock and skid conditions. For example, as the operatorof the vehicle begins to brake during regenerative braking,electromagnet energy of the motor/generator will be used to applybraking torque (i.e., electromagnetic resistance for providing torque tothe powertrain) to the vehicle. If it is determined that there is nolonger a sufficient amount of storage means to store energy recoveredfrom the regenerative braking or if the regenerative braking cannot meetthe demands of the operator, hydraulic braking will be activated tocomplete all or part of the braking action demanded by the operator.Preferably, the hydraulic braking operates in a regenerative brakeblending manner so that the blending is effectively and unnoticeablypicked up where the electromagnetic braking left off. It is desired thatthe vehicle movement should have a smooth transitional change to thehydraulic braking such that the changeover goes unnoticed by the driverof the vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a first embodiment of a brakesystem.

FIG. 2 is an enlarged schematic sectional view of the brake pedal unitassembly of the brake system of FIG. 1 shown in its rest position.

FIG. 3 is an enlarged schematic sectional view of the plunger assemblyof the brake system of FIG. 1 shown in a rest position.

FIG. 4 is an alternate embodiment of a plunger assembly which may beused in the brake system of FIG. 1.

FIG. 5 is a schematic illustration of a second embodiment of a brakesystem.

FIG. 6 is an alternate embodiment of a plunger assembly which may beused in the brake system of FIG. 5.

FIG. 7 is a schematic illustration of a third embodiment of a brakesystem.

FIG. 8 is a schematic illustration of a fourth embodiment of a brakesystem.

FIG. 9 is a schematic illustration of a fifth embodiment of a brakesystem.

FIG. 10 is an enlarged schematic sectional view of the brake pedal unitassembly of the brake system of FIG. 9 shown in its rest position.

FIG. 11 is a schematic illustration of a sixth embodiment of a brakesystem.

FIG. 12 is a schematic illustration of a seventh embodiment of a brakesystem.

FIG. 13 is a schematic illustration of an eighth embodiment of a brakesystem.

FIG. 14 is a schematic illustration of a ninth embodiment of a brakesystem.

FIG. 15 is a schematic illustration of a tenth embodiment of a brakesystem.

FIG. 16 is a schematic illustration of an eleventh embodiment of a brakesystem.

FIG. 17 is a schematic illustration of a twelfth embodiment of a brakesystem.

FIG. 18 is a schematic illustration of a thirteenth embodiment of abrake system.

FIG. 19 is a schematic illustration of a fourteenth embodiment of abrake system.

FIG. 20 is a schematic illustration of a fifteenth embodiment of a brakesystem.

FIG. 21 is a schematic illustration of a sixteenth embodiment of a brakesystem.

FIG. 22 is a schematic illustration of a seventeenth embodiment of abrake system.

FIG. 23 is an enlarged schematic illustration of the brake pedal unit ofthe brake system of FIG. 22.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, there is schematically illustrated inFIG. 1 a first embodiment of a vehicle brake system, indicated generallyat 10. The brake system 10 is a hydraulic boost braking system in whichboosted fluid pressure is utilized to apply braking forces for the brakesystem 10. The brake system 10 may suitably be used on a ground vehiclesuch as an automotive vehicle having four wheels with a wheel brakeassociated with each wheel. Furthermore, the brake system 10 can beprovided with other braking functions such as anti-lock braking (ABS)and other slip control features to effectively brake the vehicle, aswill be discussed below.

The brake system 10 generally includes a first block or brake pedal unitassembly, indicated by broken lines 12, and a second block or hydrauliccontrol unit, indicated by broken lines 14. The various components ofthe brake system 10 are housed in the brake pedal unit assembly 12 andthe hydraulic control unit 14. The brake pedal unit assembly 12 and thehydraulic control unit 14 may include one or more blocks or housingsmade from a solid material, such as aluminum, that has been drilled,machined, or otherwise formed to house the various components. Fluidconduits may also be formed in the housings to provide fluid passagewaysbetween the various components. The housings of the brake pedal unitassembly 12 and the hydraulic control unit 14 may be single structuresor may be made of two or more parts assembled together. As schematicallyshown, the hydraulic control unit 14 is located remotely from the brakepedal unit assembly 12 with hydraulic lines hydraulically coupling thebrake pedal unit assembly 12 and the hydraulic control unit 14.Alternatively, the brake pedal unit assembly 12 and the hydrauliccontrol unit 14 may be housed in a single housing. It should also beunderstood that the grouping of components as illustrated in FIG. 1 isnot intended to be limiting and any number of components may be housedin either of the housings.

The brake pedal unit assembly 12 cooperatively acts with the hydrauliccontrol unit 14 for actuating wheel brakes 16 a, 16 b, 16 c, and 16 d.The wheel brakes 16 a, 16 b, 16 c, and 16 d can be any suitable wheelbrake structure operated by the application of pressurized brake fluid.The wheel brake 16 a, 16 b, 16 c, and 16 d may include, for example, abrake caliper mounted on the vehicle to engage a frictional element(such as a brake disc) that rotates with a vehicle wheel to effectbraking of the associated vehicle wheel. The wheel brakes 16 a, 16 b, 16c, and 16 d can be associated with any combination of front and rearwheels of the vehicle in which the brake system 10 is installed. Forexample, for a vertically split system, the wheel brakes 16 a and 16 dmay be associated with the wheels on the same axle. For a diagonallysplit brake system, the wheel brakes 16 a and 16 b may be associatedwith the front wheel brakes.

The brake pedal unit assembly 12 includes a fluid reservoir 18 forstoring and holding hydraulic fluid for the brake system 10. The fluidwithin the reservoir 18 may be held generally at atmospheric pressure orcan store the fluid at other pressures if so desired. The brake system10 may include a fluid level sensor 19 for detecting the fluid level ofthe reservoir. The fluid level sensor 19 may be helpful in determiningwhether a leak has occurred in the system 10.

The brake pedal control unit assembly 12 includes a brake pedal unit(BPU), indicated generally at 20. The brake pedal unit 20 is alsoschematically shown enlarged in FIG. 2. It should be understood that thestructural details of the components of the brake pedal unit 20illustrate only one example of a brake pedal unit 20. The brake pedalunit 20 could be configured differently having different components thanthat shown in FIGS. 1 and 2.

The brake pedal unit 20 includes a housing 24 (shown broken away in FIG.2) having various bores formed in for slidably receiving variouscylindrical pistons and other components therein. The housing 24 may beformed as a single unit or include two or more separately formedportions coupled together. The housing 24 generally includes a firstbore 26, an intermediate second bore 28, and a third bore 30. The secondbore 28 has a larger diameter than the first bore 26 and the third bore30. The brake pedal unit 20 further includes an input piston 34, aprimary piston 38, and a secondary piston 40. The input piston 34 isslidably disposed in the first bore 26. The primary piston 38 isslidably disposed in the second bore 28. The secondary piston 40 isslidably disposed in the third bore 30.

A brake pedal, indicated schematically at 42 in FIGS. 1 and 2, iscoupled to a first end 44 of the input piston 34 via an input rod 45.The input rod 45 can be coupled directly to the input piston 34 or canbe indirectly connected through a coupler (not shown). The input piston34 includes an enlarged second end 52 that defines a shoulder 54. In therest position shown in FIGS. 1 and 2, the shoulder 54 of the inputpiston engages with a shoulder 56 formed between the first and secondbores 26 and 28 of the housing 24. An outer cylindrical surface 57 ofthe input piston 34 is engaged with a seal 58 and a lip seal 60 mountedin grooves formed in the housing 24. The outer cylindrical surface 57may be continuous along its length or it may be stepped having two ormore different diameter portions. The input piston 34 includes a centralbore 62 formed through the second end 52. One or more lateralpassageways 64 are formed through the input piston 34. The lateralpassageways 64 extend from the outer cylindrical surface 57 to thecentral bore 62 to provide fluid communication therebetween. The brakepedal unit 20 is in a “rest” position as shown in FIGS. 1 and 2. In the“rest” position, the pedal 42 has not been depressed by the driver ofthe vehicle. In the rest position, the passageways 64 of the inputpiston 34 are between the seals 58 and 60. In this position, thepassageways 64 are in fluid communication with a conduit 66 formedthough the housing 24. The conduit 66 is in fluid communication with aconduit 68 formed in the housing 24. The conduit 68 is in fluidcommunication with a reservoir port 70 connected to the reservoir 18. Afilter 69 may be disposed in the port 70 or the conduit 68. The conduits66 and 68 can be formed by various bores, grooves and passageways formedin the housing 24. In the rest position, the passageways 64 are also influid communication with a conduit 72 formed in the housing 24 whichleads to a simulation valve 74. The simulation valve 74 may be a cut offvalve which may be electrically operated. The simulation valve 74 may bemounted in the housing 24 or may be remotely located therefrom

The primary piston 38 is slidably disposed in the second bore 28 of thehousing 24. An outer wall 79 of the primary piston 38 is engaged with alip seal 80 and a lip seal 81 mounted in grooves formed in the housing24. The primary piston 38 includes a first end 82 having a cavity 84formed therein. A second end 86 of the primary piston 38 includes acavity 88 formed therein. One or more passageways 85 are formed in theprimary piston 38 which extend from the cavity 88 to the outer wall ofthe primary piston 38. As shown in FIG. 2, the passageway 85 is locatedbetween the lip seals 80 and 81 when the primary piston 38 is in itsrest position. For reasons which will be explained below, the passageway85 is in selective fluid communication with a conduit 154 which is influid communication with the reservoir 18.

The central bore 62 of the input piston 34 and the cavity 84 of theprimary piston 38 house various components defining a pedal simulator,indicated generally at 100. A caged spring assembly, indicated generallyat 102, is defined by a pin 104, a retainer 106, and a low ratesimulator spring 108. The pin 104 is shown schematically as being partof the input piston 34 and disposed in the central bore 62. The pin 104could be configured as a pin having a first end which is press fit orthreadably engaged with the input piston 34. The pin 104 extends axiallywithin the central bore 62 and into the cavity 84 of the primary piston38. A second end 112 of the pin 104 includes a circular flange 114extending radially outwardly therefrom. The second end 112 is spacedfrom an elastomeric pad 118 disposed in the cavity 84. The elastomericpad 118 is axially aligned with the second end 112 of the pin 104, thereason for which will be explained below. The retainer 106 of the cagedspring assembly 102 includes a stepped through bore 122. The steppedthrough bore 122 defines a shoulder 124. The second end 112 of the pin104 extends through the through bore 122. The flange 114 of the pin 104engages with the shoulder 124 of the retainer 106 to prevent the pin 104and the retainer 106 from separating from each other. One end of the lowrate simulator spring 108 engages with the second end 52 of the inputpiston 34, and the other end of the low rate simulator spring 108engages with the retainer 106 to bias the retainer 106 in a directionaway from the pin 104.

The pedal simulator 100 further includes a high rate simulator spring130 which is disposed about the pin 104. The terms low rate and highrate are used for description purposes and are not intended to belimiting. It should be understood that that the various springs of thepedal simulator 100 may have any suitable spring coefficient or springrate. In the illustrated embodiment, the high rate simulator spring 130preferably has a higher spring rate than the low rate simulator spring108. One end of the high rate simulator spring 130 engages with thebottom of the central bore 62 of the input piston 34. The other end ofthe high rate simulator spring 130 is shown in FIG. 2 in a non-engagedposition and spaced away from an end of the retainer 106. The housing24, the input piston 34 (and its seals), and the primary piston 38 (andits seals) generally define a fluid simulation chamber 144. Thesimulation chamber 144 is in fluid communication with a conduit 146which is in fluid communication with the simulation valve 74. A filter145 may be housed within the conduit 146.

As discussed above, the brake pedal unit 20 includes the primary andsecondary pistons 38 and 40 that are disposed in the second and thirdbores 28 and 32, respectively, which are formed in the housing 24. Theprimary and secondary pistons 38 and 40 are generally coaxial with oneanother. A primary output conduit 156 is formed in the housing 24 and isin fluid communication with the second bore 28. The primary outputconduit 156 may be extended via external piping or a hose connected tothe housing 24. A secondary output conduit 166 is formed in the housing24 and is in fluid communication with the third bore 30. The secondaryoutput conduit 166 may be extended via external piping or a hoseconnected to the housing 24. As will be discussed in detail below,rightward movement of the primary and secondary pistons 38 and 40, asviewing FIGS. 1 and 2, provides pressurized fluid out through theconduits 156 and 166, respectively. A return spring 151 is housed in thesecond bore 28 and biases the primary piston 38 in the leftwarddirection.

The secondary piston 40 is slidably disposed in the third bore 30. Anouter wall 152 of the secondary piston is engaged with a lip seal 153and a lip seal 154 mounted in grooves formed in the housing 24. Asecondary pressure chamber 228 is generally defined by the third bore30, the secondary piston 40, and the lip seal 154. Rightward movement ofthe secondary piston 40, as viewing FIGS. 1 and 2, causes a buildup ofpressure in the secondary pressure chamber 228. The secondary pressurechamber 228 is in fluid communication with the secondary output conduit166 such that pressurized fluid is selectively provided to the hydrauliccontrol unit 14. One or more passageways 155 are formed in the secondarypiston 40. The passageway 155 extends between the outer wall of theprimary piston 38 and a right-hand end of the secondary piston 40. Asshown in FIG. 2, the passageway 155 is located between the seal 153 andthe lip seal 154 when the secondary piston 40 is in its rest position,the reason for which will be explained below. For reasons which will beexplained below, the passageway 155 is in selective fluid communicationwith a conduit 164 which is in fluid communication with the reservoir18.

A primary pressure chamber 198 is generally defined by the second bore28, the primary piston 38, the secondary piston 40, the lip seal 81, andthe seal 153. Although the various seals shown in the drawings areschematically represented as O-ring or lip seals, it should beunderstood that they can have any configuration. Rightward movement ofthe primary piston 38, as viewing FIGS. 1 and 2, causes a buildup ofpressure in the primary pressure chamber 198. The primary pressurechamber 198 is in fluid communication with the primary output conduit156 such that pressurized fluid is selectively provided to the hydrauliccontrol unit 14.

The primary and secondary pistons 38 and 40 may be mechanicallyconnected together such that there is limited play or movement betweenthe pistons 38 and 40. This type of connection permits the primary andsecondary pistons 38 and 40 to move relative to one another byrelatively small increments to compensate for pressure and/or volumedifferences in their respective output circuits. However, under certainfailure modes it is desirable that the secondary piston 40 is connectedto the primary piston 38. For example, if the brake system 10 is under amanual push through mode, as will be explained in detail below, andadditionally fluid pressure is lost in the output circuit relative tothe secondary piston 40, such as for example, in the conduit 166, thesecondary piston 40 will be forced or biased in the rightward directiondue to the pressure within the primary chamber 1798. If the primary andsecondary pistons 38 and 40 were not connected together, the secondarypiston 40 would freely travel to its further most right-hand position,as viewing FIGS. 1 and 2, and the driver would have to depress the pedal42 a distance to compensate for this loss in travel. However, becausethe primary and secondary pistons 38 and 40 are connected together, thesecondary piston 40 is prevented from this movement and relativelylittle loss of travel occurs in this type of failure.

The primary and secondary pistons 38 and 40 can be connected together byany suitable manner. For example, as schematically shown in FIGS. 1 and2, a locking member 180 is disposed and trapped between the primary andsecondary pistons 38 and 40. The locking member 180 includes a first end182 and a second end 184. The first end 182 is trapped within the cavity88 of the second end 86 of the primary piston 38. The second end 184 ofthe locking member 180 is trapped within a recess or cavity 186 formedin the secondary piston 40. The first and second ends 182 and 184 mayinclude enlarged head portions which are trapped behind narroweropenings 192 and 194 of the cavities 88 and 186, respectively. A firstspring 188 is housed within the cavity 88 of the primary piston 38 andbiases the locking member 180 in a direction towards the primary piston38 and away from the secondary piston 40. A second spring 190 is housedwithin the cavity 186 of the secondary piston 40 and biases the lockingmember 180 in a direction towards the primary piston 38 and away fromthe secondary piston 40. The springs 188 and 190 and the locking member180 maintain the first and second output piston at a spaced apartdistance from one another while permitting limited movement towards andaway from each other by compression of the springs 188 or 190. Thislimited play mechanical connection permits the primary and secondarypistons 38 and 40 to move relative to one another by small increments tocompensate for pressure and/or volume differences in their respectiveoutput circuits.

Referring back to FIG. 1, the system 10 may further include a travelsensor, schematically shown at 240 in FIG. 1, for producing a signalthat is indicative of the length of travel of the input piston 34 whichis indicative of the pedal travel. The system 10 may also include aswitch 252 for producing a signal for actuation of a brake light and toprovide a signal indicative of movement of the input piston 34. Thebrake system 10 may further include sensors such as pressure transducers257 and 259 for monitoring the pressure in the conduits 156 and 166,respectively.

The system 10 further includes a source of pressure in the form of aplunger assembly, indicated generally at 300. As will be explained indetail below, the system 10 uses the plunger assembly 300 to provide adesired pressure level to the wheel brakes 16 a-d during a normalboosted brake apply. Fluid from the wheel brakes 16 a-d may be returnedto the plunger assembly 300 or diverted to the reservoir 18.

The system 10 further includes a first isolation valve 320 and a secondisolation valve 322 (or referred to as switching valves or base brakevalves). The isolation valves 320 and 322 may be solenoid actuated threeway valves. The isolation valves 320 and 322 are generally operable totwo positions, as schematically shown in FIG. 1. The first isolationvalve 320 has a port 320 a in selective fluid communication with theprimary output conduit 156 which is in fluid communication with thefirst output pressure chamber 198. A port 320 b is in selective fluidcommunication with a boost conduit 260. A port 320 c is in fluidcommunication with a conduit 324 which is selectively in fluidcommunication with the wheel brakes 16 a and 16 d. The second isolationvalve 322 has a port 322 a in selective fluid communication with theconduit 166 which is in fluid communication with the second outputpressure chamber 228. A port 322 b is in selective fluid communicationwith the boost conduit 260. A port 322 c is in fluid communication witha conduit 326 which is selectively in fluid communication with the wheelbrakes 16 b and 16 c.

The system 10 further includes various valves (slip control valvearrangement) for permitting controlled braking operations, such as ABS,traction control, vehicle stability control, and regenerative brakingblending. A first set of valves includes an apply valve 340 and a dumpvalve 342 in fluid communication with the conduit 324 for cooperativelysupplying brake fluid received from the boost valves to the wheel brake16 d, and for cooperatively relieving pressurized brake fluid from thewheel brake 16 d to a reservoir conduit 343 in fluid communication withthe reservoir conduit 296. A second set of valves include an apply valve344 and a dump valve 346 in fluid communication with the conduit 324 forcooperatively supplying brake fluid received from the boost valves tothe wheel brake 16 a, and for cooperatively relieving pressurized brakefluid from the wheel brake 16 a to the reservoir conduit 343. A thirdset of valves include an apply valve 348 and a dump valve 350 in fluidcommunication with the conduit 326 for cooperatively supplying brakefluid received from the boost valves to the wheel brake 16 c, and forcooperatively relieving pressurized brake fluid from the wheel brake 16c to the reservoir conduit 343. A fourth set of valves include an applyvalve 352 and a dump valve 354 in fluid communication with the conduit326 for cooperatively supplying brake fluid received from the boostvalves to the wheel brake 16 d, and for cooperatively relievingpressurized brake fluid from the wheel brake 16 d to the reservoirconduit 343.

As stated above, the system 10 includes a source of pressure in the formof the plunger assembly 300 to provide a desired pressure level to thewheel brakes 16 a-d. The system 10 further includes a venting valve 302and a pumping valve 304 which cooperate with the plunger assembly 300 toprovide boost pressure to the boost conduit 260 for actuation of thewheel brakes 16 a-d. The venting valve 302 and the pumping valve 304 maybe solenoid actuated valves movable between open positions and closedpositions. In the closed positions, the venting valve 302 and thepumping valve 304 may still permit flow in one direction asschematically shown as a check valve in FIG. 1. The venting valve 302 isin fluid communication with the reservoir conduit 296 and a first outputconduit 306 in fluid communication with the plunger assembly 300. Asecond output conduit 308 is in fluid communication between the plungerassembly 300 and the boost conduit 260.

As best shown in FIG. 3, the plunger assembly 300 includes a housing 400having a multi-stepped bore 402 formed therein. The bore 402 includes afirst portion 404, a second portion 406, and third portion 408. A piston410 is slidably disposed with the bore 402. The piston 410 includes anenlarged end portion 412 connected to a smaller diameter central portion414. The piston 410 has a second end 416 connected to a ball screwmechanism, indicated generally at 420. The ball screw mechanism 420 isprovided to impart translational or linear motion of the piston 410along an axis defined by the bore 402 in both a forward direction(rightward as viewing FIGS. 1 and 3), and a rearward direction (leftwardas viewing FIGS. 1 and 3) within the bore 402 of the housing 400. In theembodiment shown, the ball screw mechanism 420 includes a motor 422rotatably driving a screw shaft 424. The motor 422 may include a sensor426 for detecting the rotational position of the motor 422 and/or ballscrew mechanism 420 which is indicative of the position of the piston410. The second end 416 of the piston 410 includes a threaded bore 430and functions as a driven nut of the ball screw mechanism 420. The ballscrew mechanism 420 includes a plurality of balls 432 that are retainedwithin helical raceways formed in the screw shaft 424 and the threadedbore 430 of the piston 410 to reduce friction. Although a ball screwmechanism 420 is shown and described with respect to the plungerassembly 300, it should be understood that other suitable mechanicallinear actuators may be used for imparting movement of the piston 410.It should also be understood that although the piston 410 functions asthe nut of the ball screw mechanism 420, the piston 410 could beconfigured to function as a screw shaft of the ball screw mechanism 420.Of course, under this circumstance, the screw shaft 424 would beconfigured to function as a nut having internal helical raceways formedtherein.

As will be discussed in detail below, the plunger assembly 300 canprovide boosted pressure to the boost conduit 260 when actuated in boththe forward and rearward directions. The plunger assembly 300 includes aseal 440 mounted on the enlarged end portion 412 of the piston 410. Theseal 440 slidably engages with the inner cylindrical surface of thefirst portion 404 of the bore 402 as the piston 410 moves within thebore 402. A pair of seals 442 and 444 is mounted in grooves formed inthe second portion 406 of the bore 402. The seals 442 and 444 slidablyengage with the outer cylindrical surface of the central portion 414 ofthe piston 410. A first pressure chamber 450 is generally defined by thefirst portion 404 of the bore 402, the enlarged end portion 412 of thepiston 410, and the seal 440. A second pressure chamber 452, locatedgenerally behind the enlarged end portion 412 of the piston 410, isgenerally defined by the first and second portions 404 and 406 of thebore 402, the seals 442 and 444, and the central portion 414 of thepiston 410. The seals 440, 442, and 44 can have any suitable sealstructure. In one embodiment, the seal 440 is a quad ring seal. Althougha lip seal may also be suitable for the seal 440, a lip seal is moregenerally more compliant and requires more volume displacement for agiven pressure differential. This may result in a small boost pressurereduction when the piston 410 travels in the rearward direction during apumping mode.

As stated above, the brake pedal unit assembly 12 includes a simulationvalve 74 which may be mounted in the housing 24 or remotely from thehousing 24. As schematically shown in FIGS. 1 and 2, the simulationvalve 74 may be a solenoid actuated valve. The simulation valve 74includes a first port 75 and a second port 77. The port 75 is in fluidcommunication with the conduit 146 which is in fluid communication withthe simulation chamber 144. The port 77 is in fluid communication withthe conduit 72 which is in fluid communication with the reservoir 18 viathe conduits 66 and 68. The simulation valve 74 is movable between afirst position 74 a restricting the flow of fluid from the simulationchamber 144 to the reservoir 18, and a second position 74 b permittingthe flow of fluid between the reservoir 18 and the simulation chamber144. The simulation valve 74 is in the first position or normally closedposition when not actuated such that fluid is prevented from flowing outof the simulation chamber 144 through conduit 72, as will be explainedin detail below.

The following is a description of the operation of the brake system 10.FIGS. 1 and 2 illustrate the brake system 10 and the brake pedal unit 20in the rest position. In this condition, the driver is not depressingthe brake pedal 42. Also in the rest condition, the simulation valve 74may be energized or not energized. During a typical braking condition,the brake pedal 42 is depressed by the driver of the vehicle. The brakepedal 42 is coupled to the travel sensor 240 for producing a signal thatis indicative of the length of travel of the input piston 34 andproviding the signal to an electronic control module (not shown). Thecontrol module may include a microprocessor. The control module receivesvarious signals, processes signals, and controls the operation ofvarious electrical components of the brake system 10 in response to thereceived signals. The control module can be connected to various sensorssuch as pressure sensors, travel sensors, switches, wheel speed sensors,and steering angle sensors. The control module may also be connected toan external module (not shown) for receiving information related to yawrate, lateral acceleration, longitudinal acceleration of the vehiclesuch as for controlling the brake system 10 during vehicle stabilityoperation. Additionally, the control module may be connected to theinstrument cluster for collecting and supplying information related towarning indicators such as ABS warning light, brake fluid level warninglight, and traction control/vehicle stability control indicator light.

During normal braking operations (normal boost apply braking operation)the plunger assembly 300 is operated to provide boost pressure to theboost conduit 260 for actuation of the wheel brakes 16 a-d. Undercertain driving conditions, the control module communicates with apowertrain control module (not shown) and other additional brakingcontrollers of the vehicle to provide coordinated braking duringadvanced braking control schemes (e.g., anti-lock braking (AB), tractioncontrol (TC), vehicle stability control (VSC), and regenerative brakeblending). During a normal boost apply braking operation, the flow ofpressurized fluid from the brake pedal unit 20 generated by depressionof the brake pedal 42 is diverted into the internal pedal simulatorassembly 100. The simulation valve 74 is actuated to divert fluidthrough the simulation valve 74 from the simulation chamber 144 to thereservoir 18 via the conduits 146, 72, 66, and 68. Note that fluid flowfrom the simulation chamber 144 to the reservoir 18 is closed off oncethe passageways 64 in the input piston 34 move past the seal 60. Priorto movement of the input piston 34, as shown in FIGS. 1 and 2, thesimulation chamber 144 is in fluid communication with the reservoir 18via the conduits 66 and 68.

During the duration of the normal braking mode, the simulation valve 74remains open permitting the fluid to flow from the simulation chamber144 to the reservoir 18. The fluid within the simulation chamber 144 isnon-pressurized and is under very low pressures, such as atmospheric orlow reservoir pressure. This non-pressurized configuration has anadvantage of not subjecting the sealing surfaces of the pedal simulatorto large frictional forces from seals acting against surfaces due tohigh pressure fluid. In conventional pedal simulators, the piston(s) areunder increasingly high pressures as the brake pedal is depressedsubjecting them large frictional forces from the seals, therebyadversely effecting the pedal feel.

Also during the normal boost apply braking operation, the isolationvalves 320 and 322 are energized to a secondary position to prevent theflow of fluid from the conduits 156 and 166 through the valves 320 and322. Fluid flow is prevented from flowing from the ports 320 a and 322 ato the ports 320 c and 322 c, respectively. Thus, the fluid within thefirst and second output pressure chambers 198 and 228 of the brakepressure unit 20 are fluidly locked which generally prevents the firstand second output pistons 38 and 40 from moving further. Morespecifically, during the initial stage of the normal boost apply brakingoperation, movement of the input rod 45 causes movement of the inputpiston 34 in a rightward direction, as viewing FIG. 2. Initial movementof the input piston 34 causes movement of the primary piston 38 via thelow rate simulator spring 108. Movement of the primary piston 38 causesinitial movement of the secondary piston 40 due to the mechanicalconnection therebetween by the locking member 180 and the springs 188and 190. Note that during this initial movement of the primary piston38, fluid is free to flow from the primary pressure chamber 198 to thereservoir 18 via conduits 85, 154, and 68 until the conduit 85 movespast the seal 81. Also, during initial movement of the secondary piston40, fluid is free to flow from the secondary pressure chamber 228 to thereservoir 18 via the conduits 155 and 164 until the conduit 155 movespast the seal 154.

After the primary and secondary pistons 38 and 40 stop moving (byclosing of the conduits 85 and 155 and closing of the first and secondbase brake valves 320 and 322), the input piston 34 continues to moverightward, as viewing FIGS. 1 and 2, upon further movement by the driverdepressing the brake pedal 42. Further movement of the input piston 34compresses the various springs of the pedal simulator assembly 100,thereby providing a feedback force to the driver of the vehicle.

During normal braking operations (normal boost apply braking operation)while the pedal simulator assembly 100 is being actuated by depressionof the brake pedal 42, the plunger assembly 300 can be actuated by theelectronic control unit to provide actuation of the wheel brakes 16 a-d.Actuation of the isolation valves 320 and 322 to their secondarypositions to prevent the flow of fluid from the conduits 156 and 166through the valves 320 and 322 isolates the brake pedal unit 20 from thewheel brakes 16 a-d. The plunger assembly 300 may provide “boosted” orhigher pressure levels to the wheel brakes 16 a-d compared to thepressure generated by the brake pedal unit 20 by the driver depressingthe brake pedal 42. Thus, the system 10 provides for assisted braking inwhich boosted pressure is supplied to the wheel brakes 16 a-d during anormal boost apply braking operation helping reduce the force requiredby the driver acting on the brake pedal 42.

To actuate the wheel brakes 16 a-d via the plunger assembly 300 when inits rest position, as shown in FIGS. 1 and 3, the electronic controlunit energizes the venting valve 302 to its closed position, as shown inFIG. 1, such that fluid is prevented from venting to reservoir byflowing from the conduit 306 to the conduit 296. The pumping valve 304is de-energized to its open position, as shown in FIG. 1, to permit flowof fluid through the pumping valve 304. The electronic control unitactuates the motor 422 in a first rotational direction to rotate thescrew shaft 424 in the first rotational direction. Rotation of the screwshaft 424 in the first rotational direction causes the piston 410 toadvance in the forward direction (rightward as viewing FIGS. 1 and 3).Movement of the piston 410 causes a pressure increase in the firstpressure chamber 450 and fluid to flow out of the first pressure chamber450 and into the conduit 306. Fluid can flow into the boost conduit 260via the open pumping valve 304. Note that fluid is permitted to flowinto the second pressure chamber 452 via the conduit 308 as the piston410 advances in the forward direction. Pressurized fluid from the boostconduit 260 is directed into the conduits 324 and 326 through theisolation valves 320 and 322. The pressurized fluid from the conduits324 and 326 can be directed to the wheel brakes 16 a-d through openedapply valves 340, 344, 348, and 352 while the dump valves 342, 346, 350,and 354 remain closed. When the driver releases the brake pedal 42, thepressurized fluid from the wheel brakes 16 a-d may back drive the ballscrew mechanism 420 moving the piston 410 back to its rest position.Under certain circumstances, it may also be desirable to actuate themotor 422 of the plunger assembly 300 to retract the piston 410withdrawing the fluid from the wheel brakes 16 a-d. During a forwardstroke of the plunger assembly 300, the pumping valve 304 may be in itsopen position or held closed

During a braking event, the electronic control module can alsoselectively actuate the apply valves 340, 344, 348, and 352 and the dumpvalves 342, 346, 350, and 354 to provide a desired pressure level to thewheel brakes 16 d, 16 a, 16 c, and 16 b, respectively.

In some situations, the piston 410 of the plunger assembly 300 may reachits full stroke length within the bore 402 of the housing 400 andadditional boosted pressure is still desired to be delivered to thewheel brakes 16-d. The plunger assembly 300 is a dual acting plungerassembly such that it is configured to also provide boosted pressure tothe boost conduit 260 when the piston 410 is stroked rearwardly. Thishas the advantage over a conventional plunger assembly that firstrequires its piston to be brought back to its rest or retracted positionbefore it can again advance the piston to create pressure within asingle pressure chamber. If the piston 410 has reached its full stroke,for example, and additional boosted pressure is still desired, thepumping valve 304 is energized to its closed check valve position. Theventing valve 302 may be de-energized to its open position.Alternatively, the venting valve 302 may be left energized in its closedto permit fluid flow through its check valve during a pumping mode. Theelectronic control unit actuates the motor 422 in a second rotationaldirection opposite the first rotational direction to rotate the screwshaft 424 in the second rotational direction. Rotation of the screwshaft 424 in the second rotational direction causes the piston 410 toretract or move in the rearward direction (leftward as viewing FIGS. 1and 3). Movement of the piston 410 causes a pressure increase in thesecond pressure chamber 452 and fluid to flow out of the second pressurechamber 452 and into the conduit 308. Note that fluid is permitted toflow into the first pressure chamber 450 via the conduits 306 and 296 asthe piston 410 moves rearwardly or in its return stroke. Pressurizedfluid from the boost conduit 260 is directed into the conduits 324 and326 through the isolation valves 320 and 322. The pressurized fluid fromthe conduits 324 and 326 can be directed to the wheel brakes 16 a-dthrough the opened apply valves 340, 344, 348, and 352 while dump valves342, 346, 350, and 354 remain closed. In a similar manner as during aforward stroke of the piston 410, the electronic control module can alsoselectively actuate the apply valves 340, 344, 348, and 352 and the dumpvalves 342, 346, 350, and 354 to provide a desired pressure level to thewheel brakes 16 d, 16 a, 16 c, and 16 b, respectively.

As shown in FIG. 3, the first portion 404 of the bore 402 generally hasa fluid diameter D1 corresponding to where the outer diameter of theseal 440 slides along the inner cylindrical surface of the first portion404 of the bore 402. The second portion 406 of the bore 402 generallyhas a fluid diameter D2 corresponding to inner diameter of the seal 442sliding against the outer diameter of the central portion 414 of thepiston 410. The first pressure chamber 450 generally has an effectivehydraulic area corresponding to the diameter D2 since fluid is divertedthrough the conduits 306, 260, and 308 as the piston 410 is advanced inthe forward direction. The second pressure chamber 452 has an effectivehydraulic area corresponding to the diameter D1 minus the diameter D2.The plunger assembly 300 can be configured to have any suitabledimensions for the diameters D1 and D2. In one embodiment, the diametersD1 and D2 can be configured such that the effective area defined by D1can be greater than the annular effective area defined by D1 and D2.This configuration provides that on the back stroke in which the pistonis moving rearwardly, less torque (or power) is required by the motor422 to maintain the same pressure as in its forward stroke. Besidesusing less power, the motor 422 may also generate less heat during therearward stroke of piston 410. Under circumstances in which the driverpresses on the pedal 42 for long durations, the plunger assembly 300could be operated to apply a rearward stroke of the piston 410 toprevent overheating of the motor 422. Note that the chamber 450 shouldbe sized larger than the chamber 452.

Instead of using the apply valves 340, 344, 348, and 352 and the dumpvalves 342, 346, 350, and 354 to provide a desired pressure level to thewheel brakes 16 d, 16 a, 16 c, and 16 b, the system 10 could replace theapply and dump valves with single control valves (not shown) in theconduits corresponding to the wheel brakes 16 a-d. The control valvescan be actuated individually, in a multiplexing manner, between theiropen and closed positions to provide different braking pressures withinthe wheel brakes 16 a-d. This may be used during various brakingfunctions such as anti-lock braking, traction control, dynamic rearproportioning, vehicle stability control, hill hold, and regenerativebraking. Pressurized fluid is returned from the wheel brakes 16 a-d tothe plunger assembly 300 through the control valves instead of beingdiverted to the reservoir. In this situation, the plunger assembly 300is preferably configured and operated by the electronic control unit(not shown) such that relatively small rotational increments of themotor 422 and/or ball screw mechanism 420 are obtainable. Thus, smallvolumes of fluid and relatively minute pressure levels are able to beapplied and removed from the conduits associated with the wheel brakes16 a-d. For example, the motor 422 may be actuated to turn 0.5 of adegree to provide a relatively small amount of fluid and pressureincrease. This enables a multiplexing arrangement such that the plungerassembly 300 can be controlled to provide individual wheel pressurecontrol. Thus, the plunger assembly 300 and the system 10 can beoperated to provide individual control for the wheel brakes 16 a-d orcan be used to control one or more wheel brakes 16 a-d simultaneously byopening and closing the appropriate control valves (not shown).

In the event of a loss of electrical power to portions of the brakesystem 10, the brake system 10 provides for manual push through ormanual apply such that the brake pedal unit 20 can supply relativelyhigh pressure fluid to the primary output conduit 156 and the secondaryoutput conduit 166. During an electrical failure, the motor 422 of theplunger assembly 300 might cease to operate, thereby failing to producepressurized hydraulic brake fluid from the plunger assembly 300. Theisolation valves 320 and 324 will shuttle (or remain) in their positionsto permit fluid flow from the conduits 156 and 166 to the wheel brakes16 a-d. The simulation valve 74 is shuttled to its closed position 74 a,as shown in FIGS. 1 and 2, to prevent fluid from flowing out of thesimulation chamber 144 to the reservoir 18. Thus, moving the simulationvalve 74 to its closed position 74 a hydraulically locks the simulationchamber 144 trapping fluid therein. During the manual push-throughapply, the primary and secondary output pistons 38 and 40 will advancerightward pressurizing the chambers 198 and 228, respectively. Fluidflows from the chambers 198 and 228 into the conduits 156 and 166,respectively, to actuate the wheel brakes 16 a-d as described above.

During the manual push-through apply, initial movement of the inputpiston 34 forces the spring(s) of the pedal simulator to start movingthe pistons 38 and 40. After further movement of the input piston 34, inwhich the fluid within the simulation chamber 144 is trapped orhydraulically locked, further movement of the input piston 34pressurizes the simulation chamber 144 causing movement of the primarypiston 38 which also causes movement of the secondary piston 40 due topressurizing of the primary chamber 144. As shown in FIGS. 1 and 2, theinput piston 34 has a smaller diameter (about the seal 60) than thediameter of the primary piston 38 (about the seal 80). Since thehydraulic effective area of the input piston 34 is less than thehydraulic effective area of the primary piston 38, the input piston 34may travel more axially in the right-hand direction as viewing FIGS. 1and 2 than the primary piston 38. An advantage of this configuration isthat although a reduced diameter effective area of the input piston 34compared to the larger diameter effective area of the primary piston 38requires further travel, the force input by the driver's foot isreduced. Thus, less force is required by the driver acting on the brakepedal 42 to pressurize the wheel brakes compared to a system in whichthe input piston and the primary piston have equal diameters.

In another example of a failed condition of the brake system 10, thehydraulic control unit 12 may fail as discussed above and furthermoreone of the output pressure chambers 198 and 228 may be reduced to zeroor reservoir pressure, such as failure of a seal or a leak in one of theconduits 156 or 166. The mechanical connection of the primary andsecondary pistons 38 and 40 prevents a large gap or distance between thepistons 38 and 40 and prevents having to advance the pistons 38 and 40over a relatively large distance without any increase in pressure in thenon-failed circuit. For example, if the brake system 10 is under amanual push through mode and additionally fluid pressure is lost in theoutput circuit relative to the secondary piston 40, such as for examplein the conduit 166, the secondary piston 40 will be forced or biased inthe rightward direction due to the pressure within the primary chamber198. If the primary and secondary pistons 38 and 40 were not connectedtogether, the secondary piston 40 would freely travel to its furthermost right-hand position, as viewing FIGS. 1 and 2, and the driver wouldhave to depress the pedal 42 a distance to compensate for this loss intravel. However, because the primary and secondary pistons 38 and 40 areconnected together through the locking member 180, the secondary piston40 is prevented from this movement and relatively little loss of traveloccurs in this type of failure. Thus, the maximum volume of the primarypressure chamber 198 is limited had the secondary piston 40 not beconnected to the primary piston 38.

In another example, if the brake system 10 is under a manual pushthrough mode and additionally fluid pressure is lost in the outputcircuit relative to the primary piston 40, such as for example, in theconduit 156, the secondary piston 40 will be forced or biased in theleftward direction due to the pressure within the secondary chamber 228.Due to the configuration of the brake pedal unit 20, the left-hand endof the secondary piston 40 is relatively close to the right-hand end ofthe primary piston 38. Thus, movement of the secondary piston 40 towardsthe primary piston 38 during this loss of pressure is reduced comparedto a conventional master cylinder in which the primary and secondarypistons have equal diameters and are slidably disposed in the samediameter bore. To accomplish this advantage, the housing 24 of the brakepedal unit 20 includes a stepped bore arrangement such that diameter ofthe second bore 28 which houses the primary piston 38 is larger than thethird bore 30 housing the secondary piston 40. A portion of the primarychamber 198 includes an annular region surrounding a left-hand portionof the secondary piston 40 such that the primary and secondary pistons38 and 40 can remain relatively close to one another during a manualpush-through operation. In the configuration shown, the primary andsecondary pistons 38 and 40 travel together during a manual push-throughoperation in which both of the circuits corresponding to the conduits156 and 166 are intact. This same travel speed is due to the hydrauliceffective areas of the pistons 38 and 40, for their respective outputpressure chambers 198 and 228, are approximately equal. In a preferredembodiment, the area of the diameter of the secondary piston 40 isapproximately equal to the area of the diameter of the primary piston 38minus the area of the diameter of the secondary piston 40. Of course,the brake pedal unit 20 could be configured differently such that theprimary and secondary pistons 38 and 40 travel at different speeds anddistances during a manual push though operation.

During a manual push-through operation in which both of the circuitscorresponding to the conduits 156 and 166 are intact, such as during anelectrical failure described above, the combined hydraulic effectivearea of the primary and secondary pistons 38 and 40 is the area of thediameter of the primary piston 38. However, during a failure of one ofthe circuits corresponding to the conduits 156 and 166, such as by aleak in the conduit 166, the hydraulic effective area is halved suchthat the driver can now generate double the pressure within the primarychamber 198 and the non-failed conduit 156 when advancing the primarypiston 38 during a manual push-through operation via depression of thebrake pedal 42. Thus, even though the driver is actuating only two ofthe wheel brakes 16 a and 16 d during this manual push throughoperation, a greater pressure is obtainable in the non-failed primarychamber 198. Of course, the stroke length of the primary piston 38 willneed to be increased to compensate.

There is illustrated in FIG. 4 an alternate embodiment of a plungerassembly, indicated generally at 500, which may be used for the plungerassembly 300 in the brake system 10, for example. The plunger assembly500 includes a housing 502 having a multi-stepped bore 504 formedtherein. If installed into the system 10, the conduits 296, 306, and 308are in fluid communication with the bore 504. A hollow sleeve 510 may beinserted into the bore 504. Although the components of the plungerassembly 500 may be made of any suitable material, the housing 502 maybe made of aluminum for weight reduction while the sleeve 510 may bemade of a hard coat anodized metal for accepting a piston assembly 511slidably disposed therein. The sleeve 510 has a multi-stepped inner boreincluding a first portion 512, a second portion 514, and a third portion516 (similar to the first portion 404, the second portion 406, and thethird portion 408 of the bore 402 of the plunger assembly 300).

The plunger assembly 500 further includes a ball screw mechanism,indicated generally at 520. The ball screw mechanism 520 includes amotor 522 having an outer housing 523 which houses a stator 524 forrotating a rotor 526. The rotor 526 rotates a screw shaft 528 extendingalong the axis of the plunger assembly 500. A rear end of the rotor 526is supported in the housing 523 by a bearing assembly 527. The front endof the rotor 526 is connected to a multi-piece support assembly 531which is supported by a pair of bearing assemblies 533 and 535 mountedin the bore 504 of the housing 502. The bearing assemblies 527, 533, and535 are shown as ball bearing assemblies having upper and lower races.However, it should be understood that the bearings assemblies 531, 533,and 535 can be any suitable structure.

The piston assembly 511 includes a piston 530 threadably attached to anintermediate connector 532 which is threadably attached to a nut 534.The nut 534 includes an internal threaded bore 536 having helicalraceways formed therein for retaining a plurality of balls 538. Theballs 538 are also retained in raceways 540 formed in the outer surfaceof the screw shaft 528, thereby functioning as a ball screw drivemechanism. To prevent rotation of the piston assembly 511, the plunger500 can include an anti-rotation device including a pin 542 extendingradially outwardly from the intermediate connector 532. A bearingassembly 544 is attached to the pin 542 and rolls along a slot 546formed in the third portion 516 of the sleeve 510. Of course, anysuitable anti-rotation device may be used. Also, although a singleanti-rotation device is shown and described, the plunger assembly 500can have one or more, such as for example, a pair of anti-rotationdevices arranged 180 degrees apart from one another.

The piston 530 of the piston assembly 511 includes an outer cylindricalsurface 550 which sealing engages with a pair of lip seals 552 and 554mounted in grooves formed in the sleeve 510. Radial passageways 556 areformed through the sleeve 510 which are in fluid communication with thereservoir conduit 296. The piston 530 includes an enlarged end portion560 and a smaller diameter central portion 562. A seal, such as quadseal 564 is mounted in a groove formed in the enlarged end portion 560of the piston 530. The seals 552, 554, and 564 function similarly to theseals 442, 444, and 440 of the plunger assembly 300 described above.

The piston 530 of the piston assembly 511 may optionally include a stopcushion assembly, indicated generally at 570. The stop cushion assembly570 includes end member 572 connected to the end of the piston 530 by abolt 574 or other fastener. The end member 572 is disposed in a recess576 formed in the piston 530 and is mounted by the bolt 574 such thatthe end member 572 may move a limited amount relative to the piston 530.A spring member, such as a plurality of disc springs 578 (or Bellevillewasher or spring washers) bias the end member 572 in a direction awayfrom the piston 530. The right-hand most end of the end member 572, asviewing FIG. 4, extends past the end of the piston 530. The stop cushionassembly 570 provides for a cushioned stop if the end of the piston 530engages with a bottom wall 579 of the bore 504 by compression of thesprings 578.

The piston assembly 511 may also include an optional rear stop cushionassembly, indicated generally at 580. The rear stop cushion assembly 580includes a disc spring 582 disposed about the screw shaft 528 andengages with the end wall of the nut 534 of the piston assembly 511. Thedisc spring 582 may slightly compress when the piston assembly 511 ismoved back its fully rested position.

A first pressure chamber 590 is generally defined by the sleeve 510, thebore 504, the enlarged end portion 560 of the piston 530, and the seal564. A second pressure chamber 592, located generally behind theenlarged end portion 560 of the piston 530, is generally defined by thesleeve 510, the bore 504, the seals 552 and 564, and the piston 530.Passageways 594 are formed through the sleeve 510 and are in fluidcommunication with the second pressure chamber 592 and the conduit 308.

The piston assembly 500 operates in a similar manner as the plungerassembly 300 and will be described as being used in the system 10. Forexample, to actuate the wheel brakes 16 a-d when the plunger assembly500 is in its rest position, as shown in FIG. 4, the electronic controlunit actuates the motor 522 in a first rotational direction to rotatethe screw shaft 528 in the first rotational direction. Rotation of thescrew shaft 528 in the first rotational direction causes the pistonassembly 511 to advance in the forward direction (rightward as viewingFIGS. 1 and 3). Movement of the piston assembly 511 causes a pressureincrease in the first pressure chamber 590 and fluid to flow out of thefirst pressure chamber 590 and into the conduit 306. Fluid can flow intothe boost conduit 260 via the open pumping valve 304 or the check valveif the pumping valve 304 was in its closed position. Note that fluid ispermitted to flow into the second pressure chamber 592 via the conduit308 as the piston assembly 511 advances in the forward direction.Pressurized fluid from the boost conduit 260 is directed into theconduits 324 and 326 through the isolation valves 320 and 322. Thepressurized fluid from the conduits 324 and 326 can be directed to thewheel brakes 16 a-d through opened apply valves 340, 344, 348, and 352while the dump valves 342, 346, 350, and 354 remain closed. When thedriver releases the brake pedal 42, the pressurized fluid from the wheelbrakes 16 a-d may back drive the ball screw mechanism 420 moving thepiston 410 back towards its rest position.

The plunger assembly 500 is a dual acting plunger assembly such that itis configured to also provide boosted pressure to the boost conduit 260when the piston assembly 511 is stroked rearwardly. The electroniccontrol unit actuates the motor 522 in a second rotational directionopposite the first rotational direction to rotate the screw shaft 528 inthe second rotational direction. Rotation of the screw shaft 528 in thesecond rotational direction causes the piston assembly 511 to retract ormove in the rearward direction (leftward as viewing FIGS. 1 and 3).Movement of the piston 530 causes a pressure increase in the secondpressure chamber 592 and fluid to flow out of the second pressurechamber 592 and into the conduit 308. Pressurized fluid from the boostconduit 260 is directed into the conduits 324 and 326 through theisolation valves 320 and 322. The pressurized fluid from the conduits324 and 326 can be directed to the wheel brakes 16 a-d through theopened apply valves 340, 344, 348, and 352 while dump valves 342, 346,350, and 354 remain closed. The pumping valve may be closed such thatlow pressure fluid fills the first pressure chamber 590.

There is illustrated in FIG. 5 a schematic illustration of a secondembodiment of a brake system, indicated generally at 600. The brakesystem 600 is similar to the brake system 10 of FIG. 1 and, therefore,like functions and structures will not be described. Similar to thebrake system 10, the brake system 600 includes a brake pedal unit 612, ahydraulic control unit 614, and wheel brakes 616 a-d.

The brake system 600 does not include a venting valve like the ventingvalve 302 of the system 10. Instead, the brake system 600 includes aplunger assembly 620 similar to the plunger assembly 300. One of thedifferences is that the plunger assembly 620 has a piston 622 with acheck valve 624 mounted therein. The check valve 624 permits fluid toflow from a first pressure chamber 630 to a reservoir conduit 632 (incommunication with a reservoir 613) via a conduit 634 within the piston622. It is noted that the check valve 624 prevents the flow of fluidfrom the reservoir 613 to the first pressure chamber 630 via the conduit634. The check valve 624 also prevents the flow of fluid though thepiston 622 from a second pressure chamber 636 to the first pressurechamber 630.

The system 600 includes a pumping valve 640 and a check valve 642. Thecheck valve 642 is located within a conduit 644. The check valve 642restricts the flow of fluid from the first pressure chamber 630 to thereservoir 613, while permitting the flow of fluid from the reservoir 613to the pumping valve 640 and first pressure chamber 630. The pumpingvalve 640 is movable between an open position to permit the flow offluid out of the first pressure chamber 630 and to a boost conduit 650for delivering pressurized fluid to the wheel brakes 616 a-d.

When the piston 622 advances in the forward direction, rightward asviewing FIG. 5, fluid flows out of the first pressure chamber 630 andthrough the de-energized pumping valve 640 into the boost conduit 650.Note that fluid is permitted to flow into the second pressure chamber636. In the reverse stroke of the piston 622, the pumping valve 650 isenergized to a closed position and fluid flows out of the secondpressure chamber 636 but is prevented from flowing past the check valve624 into the first pressure chamber 630. Note that in a reverse stroke,the piston 622 will have been moved rightward as viewing FIG. 5 suchthat the conduit 634 is to the right of a lip seal 637 to prevent fluidflow into the reservoir 613 from the second pressure chamber 636 via theconduit 634.

One of the advantages of the brake system 600 is a reduced cost due tonot having to have a solenoid actuated venting valve. Additionally,there may not be a need to maintain power to the motor of the plungerassembly 620 on every brake apply. Another advantage is that the pumpingvalve only requires a small, low force, low cost, low current drawsolenoid since it may hydraulically latch in a closed position asindicated by the dotted line 617 in FIG. 5. Under certain situations,the system 10 may need to be controlled to de-latch the valve 640.

There is illustrated in FIG. 6 an alternate embodiment of a plungerassembly, indicated generally at 700, which may be used for the plungerassembly 620 in the brake system 600, for example. The plunger assembly700 includes a housing 702 having a multi-stepped bore 704 formedtherein. If installed into the system 600, the conduits 632, 644, and650 are in fluid communication with the bore 704. A hollow sleeve 710may be inserted into the bore 504. Although the components of theplunger assembly 700 may be made of any suitable material, the housing702 may be made of aluminum for weight reduction while the sleeve 710may be made of a hard coat anodized metal for accepting a pistonassembly 711 slidably disposed therein. The sleeve 710 has amulti-stepped inner bore including a first portion 712 and a secondportion 714. Instead of a third portion, a tube 716 is press fit or slipfit onto the end of the second portion 714. The tube 716 may be made ofan inexpensive material, such as extruded aluminum, instead of utilizingan expensive portion of the sleeve 710.

The plunger assembly 700 further includes a ball screw mechanism,indicated generally at 720. The ball screw mechanism 720 includes amotor 722 having an outer housing 723 which houses a stator 724 forrotating a rotor 726. The rotor 726 rotates a screw shaft 728 extendingalong the axis of the plunger assembly 700. The front end of the rotor526 is connected to a multi-piece support assembly 731 which issupported by generally inexpensive needle bearings (compared to moreexpensive roller angular contact ball bearings as shown in FIG. 4). Inparticular, the plunger assembly 700 includes a pair of thrust needlebearings 735 and 737 and a radial needle bearing 739. The bearingsengage with features of the support assembly 731.

The piston assembly 711 includes a piston 730 threadably attached to anut 734. The nut 734 includes an internal threaded bore 736 havinghelical raceways formed therein for retaining a plurality of balls 738.The balls 738 are also retained in raceways 740 formed in the outersurface of the screw shaft 728, thereby functioning as a ball screwdrive mechanism. To prevent rotation of the piston assembly 711, theplunger 700 can include an anti-rotation device including one or morebushings 745 that slide within corresponding slots 746 formed in thetube 716. Of course, any suitable anti-rotation device may be used.

The piston 730 of the piston assembly 711 includes an outer cylindricalsurface 750 which sealing engages with a pair of seals 752 and 754mounted in grooves formed in the sleeve 710. Radial passageways 756 areformed through the sleeve 710 which are in fluid communication with thereservoir conduit 632. The piston 730 includes an enlarged end portion760 and a smaller diameter central portion 762. A seal, such as quadseal 764 is mounted in a groove formed in the enlarged end portion 760of the piston 730.

The plunger assembly 700 may include a check valve assembly 770 locatedin the enlarged end portion 760 of the piston 730. The check valve 770is similar in function to the check valve 624 of the system 600. Thecheck valve assembly 770 includes a ball 771 selectively seated on avalve seat 772 fixed relative to the piston 730. A generally small orweak spring 773 biases the ball 771 onto the valve seat 772.

The piston assembly 711 may also include an optional rear stop cushionassembly, indicated generally at 780. The rear stop cushion assembly 780includes one or more disc spring 782 disposed about the screw shaft 728and engaged with the end wall of the nut 734 of the piston assembly 711.The disc springs 782 may slightly compress when the piston assembly 711is moved back its fully rested position.

A first pressure chamber 790 is generally defined by the sleeve 710, thebore 704, the enlarged end portion 760 of the piston 730, and the seal764. A second pressure chamber 792, located generally behind theenlarged end portion 760 of the piston 730, is generally defined by thesleeve 710, the bore 704, the seals 752 and 764, and the piston 730.Passageways 794 are formed through the sleeve 710 and are in fluidcommunication with the second pressure chamber 792 and the conduit 650.

There is illustrated in FIG. 7 a schematic illustration of a thirdembodiment of a brake system, indicated generally at 800. The brakesystem 800 is similar to the brake system 600 and, therefore, likefunctions and structures will not be described. The brake system 10 isideally suited for large passenger vehicles or trucks. Generally, largervehicles require more braking power and more fluid volume than brakesystems for smaller vehicles. This generally requires a largerconsumption of power for the motor for the plunger assembly.

The brake system 800 includes a brake pedal unit 812, a hydrauliccontrol unit 814, and wheel brakes 816 a-d. The brake assembly 800further includes a plunger assembly 820 having a piston 822 with a checkvalve 824 mounted therein. The check valve 824 permits fluid to flowfrom a first pressure chamber 830 to a reservoir conduit 832 (incommunication with a reservoir 813) via a conduit 834 within the piston822. The check valve 824 prevents the flow of fluid from the reservoir813 to the first pressure chamber 830 via the conduit 834. The checkvalve 824 also prevents the flow of fluid though the piston 882 from asecond pressure chamber 836 to the first pressure chamber 830. Thesystem 800 includes a pumping valve 840 and a check valve 842. The checkvalve 842 is located within a conduit 844. The check valve 842 restrictsthe flow of fluid from the first pressure chamber 830 to the reservoir813, while permitting the flow of fluid from the reservoir 813 to thepumping valve 840 and first pressure chamber 830. The pumping valve 840is movable between an open position to permit the flow of fluid out ofthe first pressure chamber 830 and to a boost conduit 850 for deliveringpressurized fluid to the wheel brakes 816 a-d.

Comparing the systems 600 and 800, the system 800 additionally includesa solenoid actuated quick fill valve 860. The quick fill valve 860 is influid communication with the second pressure chamber 830 via a conduit862. The quick fill valve 800 is also in fluid communication with thewheel brakes 816 a and 816 b (such as front wheel brakes) via conduit870, 872, and 874. The conduits 872 and 874 have check valves 876 and878, respectively, located therein to prevent fluid from the wheelbrakes flowing back into the conduit 870. The quick fill valve 860 mayhave relatively large orifices that enable fluid to easily flow throughthe quick fill valve 860 when in its energized to its open position,such as when the plunger assembly 820 is actuated to deliver highpressure fluid to the first pressure chamber 830. Since a lot of powermay be required to force fluid through relatively small orifices invarious valves and components of the system 800, the addition of thequick fill valve 820 helps to reduce power consumption. This isespecially useful for larger vehicles when the amount of fluid flow isincreased compared to smaller vehicles. The quick fill valve 860 may beleft energized under normal boosted braking applications. During otherevents, such as anti-lock braking or slip control, the quick fill valve820 may be moved to its closed position.

There is illustrated in FIG. 8 a schematic illustration of a fourthembodiment of a brake system, indicated generally at 900. The brakesystem 900 is similar in structure and function as the brake system 600.Instead of using a single pumping valve 640, the system 900 includes apair of pumping valves 902 and 904 in a parallel arrangement between asecond pressure chamber 910 and boost conduit 912. It may be more costeffective to provide a pair of smaller valves than a single largervalve.

There is illustrated in FIG. 9 a schematic illustration of a fifthembodiment of a brake system, indicated generally at 1000, whichincludes some of the same features as the brake systems described above.The brake system 1000 includes a brake pedal unit assembly, indicatedgenerally at 1012, and a hydraulic control unit, indicated generally at2014. The various components of the brake system 1000 are housed in thebrake pedal unit assembly 1012 and the hydraulic control unit 1014. Thebrake pedal unit assembly 1012 and the hydraulic control unit 1014 mayinclude one or more blocks or housings made from a solid material, suchas aluminum, that has been drilled, machined, or otherwise formed tohouse the various components. Fluid conduits may also be formed in thehousings to provide fluid passageways between the various components.The housings of the brake pedal unit assembly 1012 and the hydrauliccontrol unit 1014 may be single structures or may be made of two or moreparts assembled together. As schematically shown, the hydraulic controlunit 1014 is located remotely from the brake pedal unit assembly 1012with hydraulic lines hydraulically coupling the brake pedal unitassembly 1012 and the hydraulic control unit 1014. Alternatively, thebrake pedal unit assembly 1012 and the hydraulic control unit 1014 maybe housed in a single housing. It should also be understood that thegrouping of components as illustrated in FIG. 9 is not intended to belimiting and any number of components may be housed in either of thehousings.

The brake pedal unit assembly 1012 cooperatively acts with the hydrauliccontrol unit 1014 for actuating a first wheel brake 1028 a and a secondwheel brake 1028 b. The first and second wheel brakes 1028 a and 1028 bmay be, for example, located on a rear vehicle axle. Additionally, brakepedal unit assembly 1012 cooperatively acts with the hydraulic controlunit 1014 for actuating a third wheel brake 1028 c and a fourth wheelbrake 1028 d. The third and fourth wheel brakes 1028 c and 1028 d maybe, for example, located on a front vehicle axle. Each of the wheelbrakes 1028 a-d may be a conventional brake operated by the applicationof pressurized brake fluid. The wheel brake may be, for example, a brakecaliper mounted on the vehicle to engage a frictional element (such as abrake disc) that rotates with a vehicle wheel to effect braking of theassociated vehicle wheel.

As shown in FIGS. 9 and 10, the brake pedal unit assembly 1012 includesa brake pedal unit 1020 in fluid communication with a reservoir 1024.The reservoir 1024 generally holds hydraulic fluid at atmosphericpressure. The brake pedal unit 1020 includes a housing 1030 havingvarious bores formed in for slidably receiving various cylindricalpistons therein. The housing 1030 may be formed as a single unit or twoor more separately formed portions coupled together.

As best shown in FIG. 10, the housing 1030 defines a first bore 1034, acavity 1035, and a second bore 1036. The first and second bores 1034 and1036 are axially aligned with one another. As will be discussed below,an input piston 1094 is slidably disposed in the first and second bores1034 and 1036 and includes an intermediate portion disposed in thecavity 1035. A primary piston 1095 is slidably disposed in the secondbore 1036. A first port 1040 formed in the second bore 1036 is incommunication with a fluid conduit 1042. A second port 1044 formed inthe cavity 1035 is in communication with a fluid conduit 1046 incommunication with the reservoir 24.

The housing 1030 further includes a third bore 1058 and a fourth bore1060 which is narrower than the third bore 1058. As will be discussedbelow, a first secondary piston 1126 is disposed in the third and fourthbores 1058 and 1060. The housing 1030 also includes a fifth bore 1062and a sixth bore 1064 which is narrower than the fifth bore 1062. Aswill be discussed below, a second secondary piston 1127 is disposed inthe fifth and sixth bores 1062 and 1064.

The housing 1030 also includes a third port 1066 in communication with afluid conduit 1068. A fourth port 1070 and a fifth port 1072 are both incommunication with a fluid conduit 1074 which is in communication withthe reservoir 1024. A sixth port 1076 is in communication with a secondbrake fluid conduit 1078 which is in communication with the wheel brake1028 c.

The housing 1030 further includes a seventh port 1080 in communicationwith a fluid conduit 1082. An eighth port 1084 and a ninth port 1085 areboth in communication with a fluid conduit 1086 which is incommunication with the reservoir 1024. A tenth port 1088 is incommunication with a third brake fluid conduit 1090 which is incommunication with the wheel brake 28 d.

A brake pedal 1092 is coupled to a first end of the input piston 1094 ofthe brake pedal unit 1020 via an input rod 1097. The system 1000 mayfurther include one or more travel sensors 1096 for producing signalsthat are indicative of the length of travel of the brake pedal 1092. Theinput piston 1094 includes a first portion 1098 slidable within thefirst bore 1034. A seal 1100 is located between the inner surface of thefirst bore 1034 and the first portion 1098. The input piston 1094includes a second portion 1104 slidable within the second bore 1036. Aseal 1102 is located between the inner surface wall of the second bore1036 and the outer wall of the second portion 1104.

The input piston 1094 further includes an abutment portion 1116 disposedwith the cavity 1035. As will be explained below, the abutment portion1116 may at selected times (such as during a failed condition of thebrake system 1000), abut or engage with a first secondary piston 1126and a second secondary piston 1127. The abutment portion 1116 can be anysuitable feature or component integral with or connected to the inputpiston 1094 for engaging with the first and second secondary pistons1126 and 1127.

The brake pedal unit 1020 includes a pedal simulator, indicatedgenerally at 1216. The pedal simulator 1216 functions similarly as thepedal simulator 100 described above. The pedal simulator 1216 isdisposed between the input piston 1094 and a primary piston 1218slidably disposed in the bore 1036 for energizing a primary chamber 1108which is in fluid communication with the port 1040. A return spring 1118biases the primary piston 1218 towards the pedal simulator 1216. A seal1103 provides for a unidirectional seal for preventing the flow of fluidfrom escaping from the primary chamber 1108. The primary chamber 1108 isdefined by the second bore 1036, the primary piston 1218, and the seal1103.

The pedal simulator 1216 may include springs 1220 and 1222 separated bya retainer 1224 slidably disposed on a pin 1225 formed on the inputpiston 1094. An end retainer 1226 engages the spring 1222 and an end1227 of the pin 1225. Similar to the pedal simulator 100, the pedalsimulator 1216 may include an elastomeric pad 1228 axially aligned withthe end 1227 of the pin 1225. A pedal simulator chamber 1229 is definedby the seals 1102 and 1103, the input piston 1094, the primary piston1218, and the bore 1036. The pedal simulator chamber 1229 is in fluidcommunication with a conduit 1231 having a restrictive orifice 1233formed therein.

The first secondary piston 1126 includes a first end 1124 of a firstportion 1128 that steps up to a second cylindrical portion 1130. Thediameter of the second cylindrical portion 1130 is larger than thediameter of the first cylindrical portion 1128. The second cylindricalportion 1130 steps down to a third cylindrical portion 1132 of the firstsecondary piston 126. The diameter of the third cylindrical portion 1132is smaller than the second cylindrical portion 1130. A first secondarypiston spring 1134 is disposed about the circumference of the thirdcylindrical portion 1132. The ends of the first secondary piston spring1134 are disposed between a stepped surface that transitions between thesecond cylindrical portion 1130 and the third cylindrical portion 1132,and a stepped portion that transitions between the fourth bore 1058 andthe bore 1060.

The second secondary piston 1127 includes a first end 1136 of a firstportion 1140 that steps up to a second portion 1142. A diameter of thesecond portion 1142 is larger than the diameter of the first portion1140. The second portion 1142 steps down to a third portion 1144 of thesecond secondary piston 1127 that has a diameter smaller than the secondportion 1142. A second secondary piston spring 1146 is disposed aboutthe circumference of the third portion 1144. The ends of the firstsecondary piston spring 1146 are disposed between a stepped surface thattransitions between the second portion 1142 and the third portion 1144,and a stepped portion transitioning between the bore 1062 and 1064.Positioning the secondary piston springs 1134 and 1146 about thecircumference of the first secondary piston 1126 and the secondsecondary piston 1127, respectively, helps prevents each of the springsfrom buckling when compressed. In addition, the overall length of thebrake pedal unit 1020 may be reduced as in contrast to packaging therespective secondary piston springs forward of the each respectivesecondary piston.

A seal 1148 is located between the outer surface of the second portion1130 of the first secondary piston 1126 and the walls of the bore 1058.A seal 112 is located between the first portion 1128 of the firstsecondary piston 1126 and a wall of the cavity 1035. The seal 1148 andthe seal 112 seal an intermediate chamber 1150 therebetween. Similarly,a seal 1152 is located between the outer surface of the second portion1142 of the second secondary piston 1127 and the walls of the bore 1062.A seal 1114 is located between the first portion 1140 of the secondsecondary piston 1127 and a wall of the cavity 1035. The seal 1152 andthe seal 1114 seal an intermediate chamber 1154 therebetween.

A seal 1156 is located between an outer surface of the third portion1132 of the first secondary piston 1126 and the wall of the bore 1060. Afirst secondary chamber 1158 is defined by the seal 1156, the end of thefirst secondary piston 1126, and the inner walls of the bore 1060. Aseal 1160 is located between an outer surface of the third portion 1144of the second secondary piston 1127 and the wall of the bore 1064. Asecond secondary chamber 1162 is defined by the seal 1160, the end ofthe second secondary piston 1127, and the inner walls of the bore 1064.

The stepped secondary pistons 1126 and 1127 (more specifically, thethird cylindrical portions 1132 and 1144 of the first secondary piston1126 and the second secondary piston 1127, respectively) help compensatefor rear bias during normal boost braking operations and lessens dynamicrear proportioning when no electric power is present. The third portions1132 and 1144 may have smaller diameters than the respective secondportions 1136 and 1142 of each respective secondary piston. This allowsseal 1156 and seal 1160 disposed about the first and second secondarypistons 1126 and 1127, respectively, to be smaller. As the brake pedal1092 is released, the respective secondary pistons 1126 and 1127 aredragged out of their respective bores 1060 and 1064, respectively, sincethe surface area of each respective piston 1126, 1127 in contact withthe respective seals 1156 and 1160 have been reduced. As a result, lessfriction is generated as each secondary piston 1126 and 1127 slides inand out of their respective cylindrical bores 1060 and 1064.

In a preferred embodiment, the first secondary piston 1126 and thesecond secondary piston 1127 are parallel to one another and overlap oneanother. In yet another preferred embodiment, at least portions of theinput piston 1094, the first secondary piston 1126, and the secondsecondary piston 1127 are parallel to one another and overlap oneanother. As is shown in FIG. 10, the right-hand portion of the inputpiston 9104 overlaps with the left-hand portions of the first and secondsecondary pistons. The overlap of the respective pistons minimizes theoverall length (in a right to left direction as viewing FIG. 10) of thebrake pedal unit 1020 which may enhance the feasibility of packaging thebrake pedal unit in a vehicle.

Referring again to FIG. 9, the system 1000 includes a source of pressurein the form of a plunger assembly, indicated generally at 1300. Theplunger assembly 1300 may be similar in structure and function as theplunger assemblies described above. The system 1000 uses the plungerassembly 1300 to provide a desired pressure level to the wheel brakes1028 a-d during a normal boosted brake apply. Fluid from the wheelbrakes 1028 a-d may be returned to the plunger assembly 1300 or divertedto the reservoir 1024.

The system 1000 further includes a solenoid actuated simulator valve1302 movable between a closed position, as shown in FIG. 9, and an openposition when the solenoid is actuated. In the open position, thesimulator valve 1302 permits the flow of fluid between the conduit 1231to the pedal simulator chamber 1229 and a conduit 1304 which is in fluidcommunication with the reservoir 1024 via the chamber 1035 and theconduit 1046.

The system 1000 further includes a pair of solenoid actuated base brakevalves 1310 and 1312 which are each movable between an open position, asshown in FIG. 9, and a closed position when the solenoid is actuated.The pair of base brake valves 1310 and 1312 is arranged in parallel andis in fluid communication with the conduit 1042 and a conduit 1314. Itshould be understood that a single valve may be used instead of the pairof valves 1310 and 1312. It may be more cost effective to provide a pairof smaller valves than a single larger valve.

The system 1000 further includes a solenoid actuated replenishing valve1320 which is movable between a closed position, as shown in FIG. 9, andan open position when the solenoid is actuated. The replenishing valve1320 is in fluid communication with a conduit 1322 and a conduit 1324.The conduit 1324 is in fluid communication with the conduit 1314. Asolenoid actuated bypass valve 1326 is movable between an open position,as shown in FIG. 9, and a closed position when the solenoid is actuated.The bypass valve is in fluid communication with the conduit 1322 and aconduit 1328. The conduit 1328 is in fluid communication with theconduit 1068 through a check valve 1330. The conduit 1328 is also influid communication with the conduit 1082 through a check valve 1332.

Similar to the brake systems described above, the system 1000 furtherincludes various valves (slip control valve arrangement) for permittingcontrolled braking operations, such as ABS, traction control, vehiclestability control, and regenerative braking blending. A first set ofvalves includes an apply valve 1340 and a dump valve 1342 in fluidcommunication with the conduit 1324 for cooperatively supplying brakefluid to the wheel brake 1028 b, and for cooperatively relievingpressurized brake fluid from the wheel brake 1028 b to a reservoirconduit 1343 in fluid communication with the reservoir conduit 1304. Asecond set of valves include an apply valve 1344 and a dump valve 1346in fluid communication with the conduit 1324 for cooperatively supplyingbrake fluid received from the boost valves to the wheel brake 1028 a,and for cooperatively relieving pressurized brake fluid from the wheelbrake 1028 a to the reservoir conduit 1343. A third set of valvesinclude an apply valve 1348 and a dump valve 350 in fluid communicationwith a conduit 1351 for cooperatively supplying brake fluid to the wheelbrake 1028 c via conduit 1068 (as will be explained below), and forcooperatively relieving pressurized brake fluid from the wheel brake1028 c to the reservoir conduit 1343. A fourth set of valves include anapply valve 1352 and a dump valve 1354 in fluid communication with theconduit 1326 for cooperatively supplying brake fluid received from theboost valves to the wheel brake 1028 d via the conduit 1082 (as will bedescribed below), and for cooperatively relieving pressurized brakefluid from the wheel brake 1028 d to the reservoir conduit 1343.

The system 1000 may optionally include a solenoid actuated blendingvalve 1347 movable between an open position, as shown in FIG. 9, and aclosed position when the solenoid is actuated. The blending valve 1347is in fluid communication with the conduit 1314 and the conduit 1351.The blending valve 1347 may be added to the system 1000 if independentaxle regeneration blending is desired.

As stated above, the system 1000 includes a source of pressure in theform of the plunger assembly 1300 to provide a desired pressure level tothe wheel brakes 1028 a-d. The plunger 1300 may be similar to theplunger 300 and, thus, a detailed description of the plunger 1300 willnot be duplicated herein. The plunger 1300 includes a piston 1410 whichmay be moved in a forward and rearward direction. The plunger 1300defines a first pressure chamber 1450 and a second pressure chamber1452. The first pressure chamber 1450 is in fluid communication with theconduit 1322. The second pressure chamber 1452 is in fluid communicationwith the conduits 1314 and 1324. A spring biased check valve 1453prevents fluid from flowing out of the first pressure chamber 1450 butpermits the flow of fluid into the first pressure chamber 1450 via theconduit 1304 from the reservoir.

The following is a description of the operation of the brake system1000. FIGS. 9 and 10 illustrate the brake system 1000 and the brakepedal unit 1020 in the rest position. In this condition, the driver isnot depressing the brake pedal 1092. During a typical braking condition,the brake pedal 1092 is depressed by the driver of the vehicle. Thebrake pedal 1092 is coupled to the travel sensor(s) 1096 for producing asignal that is indicative of the length of travel of the input piston1094 and providing the signal to an electronic control module (notshown). The control module may include a microprocessor. The controlmodule receives various signals, processes signals, and controls theoperation of various electrical components of the brake system 1000 inresponse to the received signals. The control module can be connected tovarious sensors such as pressure sensors, travel sensors, switches,wheel speed sensors, and steering angle sensors. The control module mayalso be connected to an external module (not shown) for receivinginformation related to yaw rate, lateral acceleration, longitudinalacceleration of the vehicle such as for controlling the brake system1000 during vehicle stability operation. Additionally, the controlmodule may be connected to the instrument cluster for collecting andsupplying information related to warning indicators such as ABS warninglight, brake fluid level warning light, and traction control/vehiclestability control indicator light.

During normal braking operations (normal boost apply braking operation)the plunger assembly 1300 is operated to provide boost pressure to theconduit 1322 for actuation of the wheel brakes 1028 a-d. Under certaindriving conditions, the control module communicates with a powertraincontrol module (not shown) and other additional braking controllers ofthe vehicle to provide coordinated braking during advanced brakingcontrol schemes (e.g., anti-lock braking (AB), traction control (TC),vehicle stability control (VSC), and regenerative brake blending).

During a normal boost apply braking operation, the simulator valve 1302is actuated to its open position to divert fluid through the simulationvalve 1302 from the pedal simulator chamber 1229 to the reservoir 1204via the conduits 1304 and 1046. Note that fluid flow from the simulatorchamber 1229 to the reservoir 1204 is closed off once passageways 1065in the input piston 1094 move past the seal 1102. Prior to movement ofthe input piston 1094, as shown in FIGS. 9 and 10, the simulationchamber 1229 is in fluid communication with the reservoir 1024 via theconduits 1065 and 1046.

During the duration of the normal braking mode, the simulator valve 1302remains open permitting the fluid to flow from the simulation chamber1229 to the reservoir 1024. The fluid within the simulation chamber 1229is non-pressurized and is under very low pressures, such as atmosphericor low reservoir pressure. This non-pressurized configuration has anadvantage of reducing seal friction and force hysteresis during normallow pedal effort boosted braking, thereby improving the pedal feel.

During the duration of the normal braking mode, the base brake valves1310 and 1312 are actuated to their closed positions to prevent fluidfrom flowing out of the primary chamber 1108 of the brake pedal unit1020 via the conduit 1042. This causes the primary piston 1218 togenerally remain in a locked position permitting the springs of thepedal simulator 1216 to compress by movement of the input piston 1094providing a force feedback to the driver. The base brake valves 1310 and1312 generally isolate pressure from a boosted operation versus a manualpush through operation, as will be described below.

During normal braking operations (normal boost apply braking operation)while the pedal simulator 1216 is being actuated by depression of thebrake pedal 1092, the plunger assembly 1300 can be actuated by theelectronic control unit to provide actuation of the wheel brakes 1028a-d. The plunger assembly 1300 may provide “boosted” or higher pressurelevels to the wheel brakes 1028 a-d compared to the pressure generatedby the brake pedal unit 1020 by the driver depressing the brake pedal1092. Thus, the system 1000 provides for assisted braking in whichboosted pressure is supplied to the wheel brakes 1028 a-d during anormal boost apply braking operation helping reduce the force requiredby the driver acting on the brake pedal 1092.

To actuate the wheel brakes 1028 a-d via the plunger assembly 1300 whenin its rest position, as shown in FIGS. 9 and 10, the electronic controlunit energizes the replenishing valve 1320 to its opened position topermit flow of fluid through the replenishing valve 1320. The electroniccontrol unit actuates the motor of the plunger 1300 in a firstrotational direction to cause the piston 1410 to advance in the forwarddirection (rightward as viewing FIGS. 9 and 10). Movement of the piston1410 causes a pressure increase in the first pressure chamber 1450 andfluid to flow out of the first pressure chamber 1450 and into theconduit 1322. The pressurized fluid in the conduit 1322 flows throughthe open replenishing valve 1320 and into the conduit 1324. Thepressurized fluid within the conduit 1324 actuates the wheel brakes 1028a and 1028 b through the open apply valves 1342 and 1340. Note that thedump valves 1346 and 1342 are closed preventing fluid from venting intothe reservoir 1024.

To actuate the wheel brakes 1028 c and 1028 d, pressurized fluid fromthe first pressure chamber 1450 can be directed into the conduits 1068and 1082 via a couple of different pathways. Note that fluid flowinginto the conduits 1068 and 1082 will actuate the wheel brakes 1028 c and1028 d through the brake pedal unit 1020, as will be described below. Ina first pathway, fluid flows out of the first pressure chamber 1450,through the conduit 1322, through the replenishing valve 1320, throughthe conduit 1324, and through the open optional axle blending valve 1347into the conduit 1351. Pressurized fluid in the conduit 1351 can thenflow through the open apply valves 1348 and 1352 and into the conduits1068 and 1082. In a second pathway, fluid flows out of the firstpressure chamber 1450, through the conduit 1322 through the bypass valve1326, through the conduit 1328, through the one way check valves 1330and 1332, and into the conduits 1068 and 1082. Providing two pathwaysmay be beneficial during a spike apply in which the flow of fluid intothe conduits 1068 and 1082 very rapidly with a relatively large amountof fluid may be desirable. Since the orifices and flow paths within theapply valves 1348 and 1352 may be relatively small to provide a moreefficient pressure modulation during slip control, the second pathwaymay provide a beneficial additional flow path into the conduits 1068 and1082.

Pressurized fluid from the conduits 1068 and 1082 can be directed to thewheel brakes 1028 c and 1028 d, respectively, via the brake pedal unit1020. More specifically, the pressurized fluid within the conduits 1068and 1082 expands the intermediate chambers 1150 and 1154, respectively,of the brake pedal unit 1020. The pressurized hydraulic brake fluidentering the intermediate chambers 1150 and 1154 exerts a force on thefirst secondary piston 1126 and the second secondary piston 1127,respectively. The exerted force on the first and second secondarypistons 1126 and 1127 pressurizes the brake fluid in the first secondarychamber 1158 and the second secondary chamber 1162. The pressurizedhydraulic brake fluid in the first secondary chamber 1158 is in fluidcommunication with wheel brake 1028 c via the second brake fluid conduit1078. Similarly, the pressurized hydraulic brake fluid in the secondsecondary chamber 1162 is in fluid communication with wheel brake 1028 dvia the third conduit 1090. During release of the brake pedal 1092,fluid may flow in the reverse direction than described above.

During a braking event, the electronic control module can alsoselectively actuate the apply valves 1340, 1344, 1348, and 1352 and thedump valves 1342, 1346, 1350, and 1354 to provide a desired pressurelevel to the wheel brakes 1028 a-d.

In some situations, the piston 1410 of the plunger assembly 300 mayreach its full stroke length and additional boosted pressure is stilldesired to be delivered to the wheel brakes 1028 a-d. Similar to theplunger assembly 300, the plunger assembly 1300 may be a dual actingplunger assembly such that it is configured to also provide boostedpressure to the second pressure chamber 1452 when the piston 1410 isstroked rearwardly. In this situation, the replenishing valve 1320 isactuated to its closed position. Pressurized fluid from the conduit 1314is directed through the apply valves 1340 and 1344 to the wheel brakes1028 b and 1028 a. For actuation of the wheel brakes 1028 c and 1028 d,pressurized fluid from the conduit 1314 through the blending valve 1347following the first pathway as described above. Note that the bypassvalve 1312 may be actuated to its closed position, such as during a slipcontrol operation. Fluid flows into the expanding first pressure chamber1450 from the reservoir 1024 via the conduit 1304 and the check valve1453. In a similar manner as during a forward stroke of the piston 1410,the electronic control module can also selectively actuate the applyvalves 1340, 1344, 1348, and 1352 and the dump valves 1342, 1346, 1350,and 1354 to provide a desired pressure level to the wheel brakes 1028a-d.

In the event of an electrical brake failure or possibly some otherfailure, such as a leak, the brake system 1000 provides for manualbraking or a manual push though operation. For example, during anelectrical failure, the motor of the plunger assembly 1300 might ceaseto operate, thereby failing to produce pressurized hydraulic brake fluidat the first pressure chamber 1450. To provide manual braking, thedriver exerts a higher force on the brake pedal 1092. The simulatorvalve 1302 is in its non-energized closed position. This essentiallyhydraulically locks the pedal simulator chamber 1229 preventing fluidfrom leaving the pedal simulator chamber 1229. Thus, the springs 1220and 1222 of the pedal simulator 1300 will not compress. Movement of theinput piston 1094 will cause movement of the primary piston 1218 due tothe locked pedal simulator chamber 1229. The base brake valves 1310 and1312 are in their non-energized open positions. This providespressurized fluid from the primary chamber 1108 to flow through the basebrake valves 1310 and 1312 into the conduit 1314. The wheel brakes 1028a and 1028 b can be actuated by the flow of fluid through the applyvalves 1340 and 1344. Fluid flow may also be directed into the conduits1068 and 1082. Additionally, if a leak occurs in the conduits 1068,1082, 1078, or 1090, during a manual push through operation the inputpiston 1094 can be used to manually push one or both of the first andsecond secondary pistons 1126 and 1127 in a right-ward direction, asviewing FIGS. 9 and 10. This will actuate one or both of the wheelbrakes 1028 c and 1028 d. To apply manual push through for braking thefront wheel brakes 1028 c and 1028 d, the driver may exert a generallylonger travel on the brake pedal 1092. The longer travel displaces theinput piston 1094 beyond the range used during normal boost operation.In this situation, the abutment portion 1116 of the input piston 1094contacts the left-hand end portions of the first and second secondarypistons 1126 and 1127, respectively, as viewing FIG. 10. Thus, asviewing FIG. 10, rightward movement of the input piston 1094 will drivethe secondary pistons 1126 and 1127 in the rightward direction. As thefirst and second secondary pistons 1126 and 1127 are displaced, brakefluid within the first and second secondary chambers 1158 and 1162 ispressurized, thereby exerting a force for actuating the front wheelbrakes 1028 c and 1028 d. via the conduits 1078 and 1090, respectively.In the event leakage occurs in one of the secondary chambers, one of thefront wheel brakes 1028 c or 1028 d may be used for braking since both(front) wheel brakes 1028 c and 1028 d are independently actuatable.Manual braking will be available for the rear wheel brakes 1028 a and1028 b and for the respective front wheel brake 1028 c or 1028 d thatmaintains its hydraulic brake fluid conduit integrity.

Regenerative braking is typically applied to one of the respective axlesof a vehicle for energy recapture by simultaneously reducing pressurewhile exerting an electromagnetic resistive force to the axle. Duringperiods of braking when regenerative braking is applied to a respectiveaxle for maximum recapture of energy, brake blending may occur so thatthe regenerative braking being applied to the respective axle does notcreate a torque imbalance between each axle of the vehicle. Too muchwheel torque in a respective region of the vehicle may lead to a wheelslip condition. The optional blending valve 1347 is provided to assistduring regenerative braking operations. For example, if regenerativebraking is performed on the rear axle, it may be desirable to applyadditional pressure to the front wheel brakes 1028 c and 1028 d tocompensate. The blending valve 1347 can be energized accordingly toisolate the front wheel brake circuit from the rear wheel brake circuit.The plunger assembly 1300 can be actuated accordingly to provide adesired pressure level on each of the brake circuits by isolating thedesired circuit by actuation of the blending valve 1347 and thenpermitting an increase in pressure by the plunger assembly 1300.

There is illustrated in FIG. 11 an alternate embodiment of a brakesystem, indicated generally at 1500. The brake system 1500 is similar tothe brake system 1000 described above with respect to FIGS. 9 and 10and, therefore, similar features will not be described in duplicate. Oneof the differences is that the brake system 1500 uses only a single basebrake valve 1502 instead of a pair of parallel arranged valves 1310 and1312, as described above with respect to the brake system 1000. The useof a single valve may be used instead of a pair of valves to improvepackaging space, weight or cost. Another difference between the brakesystems 1000 and 1500 is that the brake system 1500 does not include theuse of a blending valve, such as the optional blending valve 1347 of thebrake system 1000.

There is illustrated in FIG. 12 an alternate embodiment of a brakesystem, indicated generally at 1600. The brake system 1600 is similar tothe brake system 1000 described above with respect to FIGS. 9 and 10and, therefore, similar features will not be described in duplicate.Unlike the brake system 1500, the brake system 1600 includes a blendingvalve 1602 and a pair of base brake valves 1604 and 1606 arranged in aparallel manner.

Another difference is that the brake system 1600 does not include apedal simulator within a brake pedal unit 1610, but rather includes apedal simulator, indicated generally at 1620, located remotely from thebrake pedal unit 1620. Instead of being mechanically actuated by aninput piston 1622 of the brake pedal unit 1620, the pedal simulator 1620is hydraulically actuated. Movement of input piston 1622 energizes apressure chamber 1624 which is in fluid communication with a conduit1626. The conduit 1626 is in fluid communication with a solenoidactuated simulator valve 1628 which is movable between a closedposition, as shown in FIG. 12, and an open position when actuated by thesolenoid. The pedal simulator 1620 includes a housing 1630 having a bore1632 formed therein. A piston 1634 is slidable disposed in the bore 1632and sealing engages with a seal 1636. A pressure chamber 1638 is definedby the bore 1632, the piston 1634, and the seal 1636. The pressurechamber 1638 is in fluid communication with a conduit 1640 having arestricted orifice 1642 formed therein. The conduit 1640 is also influid communication with the simulator valve 1628. The pedal simulator1620 has a caged spring design having a pair of springs 1650 and 1652which may have different spring rates. A separating member 1654 engageswith and separates the springs 1650 and 1652. The piston 1634 includesan outwardly extending pin 1656 engaged with a retainer 1658. The pedalsimulator 1620 may also include an elastomeric pad 1660 which engageswith an end 1662 of the pin 1656 after sufficient travel of the piston1634. Compression of the elastomeric pad 1660 by the end 1662 of the pin1656 may provide a different spring rate characteristic of the pedalsimulator 1620 at this point of travel.

During a normal boosted operation of the brake system 1600, the inputpiston 1622 is advanced by a brake pedal 1623. Movement of the inputpiston 1622 pressurized the chamber 1624 and the conduit 1626. Thesimulator valve 1628 is actuated to its open position, therebypermitting the flow of fluid through the simulator valve 1628, theconduit 1640, and the orifice 1642. The flow of pressurized fluid entersinto the chamber 1638 advancing the piston 1634 and compressing thesprings 1650 and 1652, which provide a force feedback to the driver.Note that a check valve 1643 arranged parallel to the conduits 1626 and1640 prevents the flow of fluid around the simulator valve 1628, butpermits the flow of fluid around the simulator valve 1628 when the flowof fluid is in the direction from the chamber 1638 back into the chamber1624.

There is illustrated in FIG. 13 an alternate embodiment of a brakesystem, indicated generally at 1700. The brake system 1700 is similar tothe brake system 600 described above with respect to FIG. 5. One of thedifferences is that the brake system 600 includes a bypass valve 1702which functions in a similar manner as the bypass valve 1326 of thebrake system 1000 described above with respect to FIG. 9. The bypassvalve 1702 provides a secondary pathway via conduits 1704 and 1706 forpressurizing wheel brakes 1710 and 1712. Check valves 1720 and 1722 areprovided in the conduit 1706. The check valves 1720 and 1722 operated ina similar manner as the check valves 1330 and 1332 of the brake system1000 in FIG. 9 as described above.

The brake system 1700 may be configured as a diagonally split system inwhich wheel brakes associated with opposite corner wheels are in onebrake circuit and the other opposed corner wheel brakes are in anothercircuit. For example, the wheel brake 1710 may be associated with aright front wheel and a wheel brake 1730 may be associated with a leftrear wheel. Fluid from a conduit 1740 from an exit port of a firstisolation valve 1742 is in fluid communication with the wheel brakes1710 and 1730. The wheel brake 1712 may be associated with a left frontwheel and a wheel brake 1732 may be associated with a right rear wheel.Fluid from a conduit 1744 from an exit port of a first isolation valve1746 is in fluid communication with the wheel brakes 1712 and 1732.

The brake system 1700 includes a brake pedal unit, indicated generallyat 1750. A pedal simulator 1752 and simulator valve 1754 are locatedremotely from the brake pedal unit 1750. The pedal simulator 1752 andthe simulator valve 1754 functions in a similar manner as the pedalsimulator 1620 and the simulator valve 1628 of the brake system 1600described above with respect to FIG. 12. The brake pedal unit 1750includes an input piston 1760, a primary piston 1762, and a secondarypiston 1764. Excluding the pedal simulator features, the brake pedalunit 1750 operates in a similar manner as the brake pedal unit 20 of thebrake system 10 described above with respect to FIG. 1. One of thedifferences is that portions of the input piston 1760 overlaps portionsof the primary piston 1762 in a radial direction. More specifically, theinput piston 1760 includes a tubular extension 1770 which extends into atubular extension 1772 of the primary piston 1762 by a distance D whenthe brake pedal unit 1750 is at rest, as shown in FIG. 13. Thisoverlapping configuration helps to reduce the overall length of thebrake pedal unit 1750 to provide a packaging advantage when installedinto a vehicle's engine compartment. The reduction in length is providedby the overlapping distance D as well as not having to have an initialgap between the pistons.

There is illustrated in FIG. 14 an alternate embodiment of a brakesystem, indicated generally at 1800. The brake system 1800 is similar tothe brake system 1700 described above with respect to FIG. 13. One ofthe differences is that the brake system 1800 may be configured as avertically split system. For example, a wheel brake 1802 may beassociated with a right front wheel, a wheel brake 1804 may beassociated with a left front wheel, a wheel brake 1806 may be associatedwith a right rear wheel, and a wheel brake 1808 may be associated with aleft rear wheel. The front wheel brakes 1802 and 1804 are on one fluidcircuit and the rear wheel brakes 1806 and 1808 are on another fluidcircuit. It should be understood that any of the brake systems describedherein can be configured as a diagonally split system, a verticallysplit system, or any other configuration in which the wheel brakes areassociated with desired wheel placements. In a split configurationsystem, the fluid of one fluid circuit does not mix with the fluid ofanother fluid circuit in manual push-through mode. A split configurationmay also help to assure that if one of the fluid circuits has acatastrophic failure, such as a leak or component failure, that thewheel brakes of the other circuit will still be operable.

The brake system 1800 includes a solenoid actuated blending valve 1820which operates in a similar manner as the blending valve 1347 of thebrake system 1000 described above with respect to FIG. 9. The blendingvalve 1820 may be added to the system 1800 if independent axleregeneration blending is desired on the rear wheels for example.

There is illustrated in FIG. 15 an alternate embodiment of a brakesystem, indicated generally at 2000. The brake system 2000 is similar tothe brake system 1700 described above with respect to FIG. 13. The brakesystem 2000 includes a brake pedal unit 2002, a pedal simulator 2004, aplunger assembly 2006, and a reservoir 2008 which may be similar instructure and function as described above with respect to otherembodiments of brake systems described herein. One of the differences isthat the brake system 2000 includes an optional simulator test valve2010 which may be electronically controlled between an open position, asshown in FIG. 15, and a powered closed position. The simulator testvalve 2010 is not necessarily needed during a normal boosted brake applyor for a manual push through mode. The simulator test valve 2010 can beactuated to a closed position during various testing modes to determinethe correct operation of other components of the brake system 2000. Forexample, the simulator test valve 2010 may be actuated to a closedposition to prevent venting to the reservoir 2008 via a conduit 2012such that a pressure build up in the brake pedal unit 2002 can be usedto monitor fluid flow to determine whether leaks may be occurringthrough seals of various components of the brake system 2000.

There is illustrated in FIG. 16 an alternate embodiment of a brakesystem, indicated generally at 2100. The brake system 2100 is similar tothe brake system 2000 described above with respect to FIG. 14. The brakesystem 2100 includes a brake pedal unit 2102, a pedal simulator 2104, aplunger assembly 2106, a reservoir 2108, and a simulator test valve 2110which may be similar in structure and function as described above withrespect to other embodiments of brake systems described herein. One ofthe differences is that the brake pedal unit 2102 includes a steppedinput piston 2120 compared to the single diameter input piston 2020 ofthe brake pedal unit 2002 in FIG. 15. The stepped input piston 2120includes a large diameter portion 2122 and a small diameter portion2124. Leftward motion of the stepped input piston 2120 causes the smalldiameter portion 2124 to pressurize a first fluid chamber 2126 in whichthe flow of fluid is diverted from the first fluid chamber 2126 into thepedal simulator 2104 via a conduit 2128 and a simulation valve 2130 whenin its opened position. This flow path and actuation of the pedalsimulator 2104 is similar to the operation of the brake systemsdescribed above. However, the brake system 2100 includes an additionalquick fill or fast fill feature utilizing the input stepped piston 2120such as during a manual push through operation. The structure andfunction of the stepped input piston 2120 may be similar to the steppedpiston design shown and described in U.S. Pat. No. 5,557,935, which isincorporated by reference herein and attached hereto. This stepped inputpiston design provides a quick fill functionality that will reducemanual apply pedal travel and increase the available pressure for agiven pedal force. The fast fill feature also utilizes a proportioningvalve 2132 similar in operation and structure as the valve body 42 shownand described in U.S. Pat. No. 5,557,935. During actuation, pressurewithin an annular fast-fill pressurizing chamber 2142 of the brake pedalunit 2102 is increased due to leftward movement of the large diameterportion 2122. Fluid flows out of the fast-fill pressurizing chamber 2142and regulated by the proportioning valve 2132 where fluid flow may bepermitted to flow to the wheel brakes via a conduit 2144. Theproportioning valve 2132 may meter the flow instead of causing an abruptchange in pressure. The proportioning valve 2132 may be configured todivert flow until a desired pressure level, such as around 7 bar. Forexample, when the pressure within the conduit 2144 reaches around 7 bar(or some other predetermined pressure), the proportioning valve 2132vents the fast-fill pressurizing chamber 2142 to reservoir.

An L-type fixed seal 2150 may be replaced by a larger diameter L-typepiston seal on the stepped piston 2120. This may provide adequate flowthrough the seal 2150 during a manual push through operation providingflow from the fast-fill pressurizing chamber 2142 into the first fluidchamber 2126 to advance the pistons of the brake pedal unit 2102leftward during a manual push through event.

There is illustrated in FIG. 17 an alternate embodiment of a brakesystem, indicated generally at 2200. The brake system 2200 is similar tothe brake system 2000 described above with respect to FIG. 14. The brakesystem 2200 includes a brake pedal unit 2202, a pedal simulator 2204, aplunger assembly 2206, and a reservoir 2208, which may be similar instructure and function as described above with respect to otherembodiments of brake systems described herein. One of the differences isthat the plunger assembly 2206 may include redundant control featuressuch that the plunger assembly 2206 may be electrically controlled byanother source, such as a secondary actuator (motor, stator, or coil,for example), indicated schematically at 2220 (outer), in addition to aprimary motor, indicated generally at 2222 (inner). The inclusion of thesecondary actuator 2220 adds redundancy to the brake system 2200 suchthat in case of a failure of the primary motor 2222, the secondaryactuator 2220 may be actuated to control the plunger assembly 2206. Thebrake system 2200 may also include a redundant travel sensor 2230 todetect pedal travel and a redundant sensor 2232 to detect movementand/or position of the piston of the plunger assembly 2206. Thesecondary actuator 2220, the travel sensor 2230, and the sensor 2232 maybe on a separate electrical circuit than the rest of the brake system2200.

There is illustrated in FIG. 18 an alternate embodiment of a brakesystem, indicated generally at 2300. The brake system 2300 is similar tothe brake systems 2000 and 2200 described above. The brake system 2300includes a brake pedal unit 2302, a pedal simulator 2304, a plungerassembly 2306, and a reservoir 2308, which may be similar in structureand function as described above with respect to other embodiments ofbrake systems described herein. One of the differences is that the brakesystem 2300 includes a secondary source 2330 having a motor 2332 and apump assembly 2334 to provide a back-up boost function for improvedmanual push through operation, especially when the driver is not pushingon the brake pedal and therefore not providing fluid pressure at thebrake pedal unit 2302. Under these conditions, the motor 2332 of thesecondary source 2330 may be activated to drive the pump assembly 2334to provide pressurized fluid to a conduit 2336 for advancing the pistons2340 and 2342 of the brake pedal unit 2302. Advancement of the pistons2340 and 2342 provides pressurized fluid the wheel brakes in a similarmanner as the manual push through operations described above withrespect the other brake systems in which the plunger assembly isbypassed. While the secondary source 2330 may add cost to the brakesystem 2300, the secondary source 2330 provides the ability to providebraking pressure to the wheel brakes in the event of a failure of theplunger assembly 2306 and when the driver is not pushing on the brakepedal. This brake system 2300, with the inclusion of the secondarysource 2330, may also be used for fully autonomous vehicles whereinbraking may be desired even through the driver is not operating thebrake pedal. In this situation, the plunger assembly 2306 or thesecondary source 2330 may be operated to provide pressure to the wheelbrakes. In the event that the plunger assembly 2306 is experiencing afailed condition, the secondary source 2330 may be actuated. It may bedesirable to have the motor 2332, the simulator test valve 2333, and aredundant pedal travel sensor 2335 be connected to a separate powersupply such that the secondary source 2330 may be actuated even in theevent of an electrical failure of the primary electrical circuit of thebrake system 2300. The secondary source 2330 may also be actuated evenif the driver is applying force to the brake pedal to provide an evengreater pressure increase within the brake pedal unit 2302.

There is illustrated in FIG. 19 an alternate embodiment of a brakesystem, indicated generally at 2400. The brake system 2400 is similar tothe brake systems described above. The brake system 2400 includes abrake pedal unit 2402, a pedal simulator 2404, a plunger assembly 2406,and a reservoir 2408, which may be similar in structure and function asdescribed above with respect to other embodiments of brake systemsdescribed herein. Note that the brake pedal unit 2402 may be simplifiedcompared to the previously shown and described brake pedal units suchthat the pistons of the brake pedal unit 2402 are not stepped, therebyreducing the cost of the brake pedal unit 2402. One of the differencesis that various components of the brake system 2400 may be included in asecond housing, indicated generally by broken lines 2410, which may belocated remotely from the remaining components of the brake system 2400.This arrangement may provide packaging improvements. The brake system2400 includes a back-up boost function for improving manual push throughperformance in the form of a secondary source, indicated generally at2440. The secondary source 2440 includes a motor 2442 and a pair ofpumps 2444 and 2446 for each of the two brake circuits of the brakesystem 2400. Similar to brake system 2300, the secondary source 2440 mayprovide pressurized fluid to the wheel brakes during a failed conditionof the plunger assembly 2406 and in the situation in which the driver isnot pressing on the brake pedal. The brake system 2400 may include fluidaccumulators 2450 and 2452 connected to inlets of the pumps 2444 and2446. It may be desirable to have the motor 2332 and the solenoidactuated valves housed within the second housing 2410 to be connected toa separate power supply such that the secondary source 2440 may beactuated even in the event of an electrical failure of the primaryelectrical circuit of the brake system 2400. The brake system 2400 mayinclude valves 2480 and 2482 for providing a secondary flow path for aspike apply event. Alternatively, the plunger assembly 2406 may beeliminated from the brake system 2400 such that the secondary source2440 may provide normal boosted braking.

There is illustrated in FIG. 20 an alternate embodiment of a brakesystem, indicated generally at 2500. The brake system 2500 is similar tothe brake system 2400 described above. The brake system 2500 includes abrake pedal unit 2502, a pedal simulator 2504, a plunger assembly 2506,a reservoir 2508, and a secondary source 2540. One of the differences isthat the brake system 2500 includes a bypass valve 2580 and 2582 foreach brake circuit. Each bypass valve 2480 and 2482 is arranged inparallel to a respective isolation valve 2490 and 2492 to provide asecondary flow path when the bypass valves 2580 and 2582 are in theiropen positions. This secondary flow path may be useful if the physicalstructure of the isolation valves do not permit adequate flow throughthe open isolation valves. It may be cost prohibitive to design suchisolation valves that provide the desired flow through such as during anautonomous braking event. Therefore, the inclusion of the bypass valves2580 and 2582 may be less expensive than designing isolation valves(less restrictive orifices) capable of providing a desired flowtherethrough.

There is illustrated in FIG. 21 an alternate embodiment of a brakesystem, indicated generally at 2600. The brake system 2600 is similar tothe brake systems 2000 and 2100 shown and described with respect toFIGS. 15 and 16, respectively. The brake system 2600 includes a brakepedal unit 2602, a pedal simulator 2604, a plunger assembly 2606, and areservoir 2608 which may be similar in structure and function asdescribed above with respect to other embodiments of brake systemsdescribed herein. The brake system 2600 also includes a fast fill valve2610. The fast fill valve 2610 may operate in a similar manner as thesimulator test valve 2010 described above with respect to the brakesystem 2000 such that the fast fill valve 2610 may be electronicallycontrolled between an open position, as shown in FIG. 21, and a closedposition. The fast fill valve 2610 may be actuated to a closed positionduring various testing modes to determine the correct operation of othercomponents of the brake system 2000.

In addition, the fast fill valve 2610 may further be used to perform afast fill function as described above with respect to the brake system2100. For example, the fast fill valve 2610 may be used in place of theproportioning valve 2132 of the brake system 2100. The fast fill valve2610 may be configured to be operated in an electronicallyproportionally controlled manner and not merely a digital type on/offvalve. Thus, the pressure and/or flow rate through the fast fill valve2610 may be controlled between its extreme open and closed positions.Similar to the brake system 2100, the brake pedal unit 2602 of the brakesystem 2600 includes a stepped input piston 2620 having a large diameterportion 2622 and a small diameter portion 2624. Leftward motion of thestepped input piston 2620 causes the small diameter portion 2624 topressurize a first fluid chamber 2626 in which the flow of fluid isdiverted from the first fluid chamber 2626 into the pedal simulator 2604via a conduit 2628 and a simulation valve 2630 when in its openedposition. This flow path and actuation of the pedal simulator 2104 issimilar to the operation of the brake systems described above during anormal boosted event. The fast fill valve 2610 may be energized to itsclosed position permitting flow from an annular fast-fill pressurizingchamber 2642 of the brake pedal unit 2602 to the first fluid chamber2626. During a manual push through operation, an L-type fixed seal 2650may be replaced by a larger diameter L-type piston seal on the steppedpiston 2620 to provide adequate flow through the seal 2150 from thefast-fill pressurizing chamber 2642 into the first fluid chamber 2626 toadvance the pistons of the brake pedal unit 2102 leftward. The fast fillvalve 2610 can be proportionally controlled to vent to the fast fillpressurizing chamber 2642 to the reservoir 2608 at a desired pressurelevel.

The electronic control unit for the brake system 2600 may be configuredto learn the pressure-volume relationship of the wheel brakes from theelectronic control unit of the plunger assembly 2606. Based on thisinformation and knowing the various piston diameters, orifice sizes,etc., the fast fill valve 2610 can be controlled based on input travel.The pressure control of the fast fill valve 2610 may be adjusted basedon the driver's apply rate. The plunger assembly 2606 could be used tocalibrate the control of the fast fill valve 2610 based on pressurefeedback. During a fast fill mode, the fast fill valve 2610 may becontrolled in an open loop manner. Input travel (brake pedal travel orpiston 2620 travel) may be detected and monitored by a travel sensor2670. A secondary travel sensor 2672 may additionally be used which maybe connected in a different electrical circuit along with the control ofthe solenoid of the fast fill valve 2610 to function as a redundant orback up in case of electrical failure of the main circuit.

There is illustrated in FIG. 22 an alternate embodiment of a brakesystem, indicated generally at 2700. The brake system 2700 is similar tothe brake system 2600 described above, which may be similar in operationand structure to other brake systems described above. Other features ofbrake systems described above may be included in the brake system 2700.

The brake system 2700 includes a brake pedal unit, indicated generallyat 2702, a reservoir 2704, a pedal simulator 2706, and a simulationvalve 2708, all of which may function in a similar manner as describedabove with respect to the other brake systems.

The brake system 2700 further includes a plunger assembly, indicatedgenerally at 2710, a first plunger valve 2712, and a second plungervalve 2714. The plunger assembly 2710 may be a dual acting plungerassembly 2710 such that it is configured to also provide pressurizedfluid to the brake system 2700 when a piston 2716 of the plungerassembly 2710 is stroked rearwardly as well as forwardly. However, it iswithin the scope of the invention that the plunger assembly 2710 can bea single acting plunger assembly such that is it configured to providepressurized fluid to the brake system 2700 when the piston 2716 of theplunger assembly 2710 is stroked on in the rearward or forwarddirection. The first plunger valve 2712 is preferably a solenoidactuated normally closed valve. Thus, in the unactuated state, the firstplunger valve 2712 is in a closed position, as shown in FIG. 22. Thesecond plunger valve 2714 is preferably a solenoid actuated normallyopen valve. Thus, in the unactuated state, the second plunger valve 2714is in an open position, as shown in FIG. 22. A check valve may bearranged within the second plunger valve 2714 so that when the secondplunger valve 2714 is in its closed position, fluid may still flowthrough the second plunger valve 2714 in the direction from a firstoutput conduit 2715 (from a first pressure chamber 2716 of the plungerassembly 2710) to a conduit 2717 leading to the isolation valves 2720and 2722 (see below). Note that during a rearward stroke of the plungerassembly 2710, pressure may be generated in a second pressure chamber2718 for output into the conduit 2717.

Generally, the first and second plunger valves 2712 and 2714 arecontrolled to permit fluid flow at the outputs of the plunger assembly2710 and to permit venting to the reservoir 2704 through the plungerassembly 2710 when so desired. For example, the first plunger valve 2712may be energized to its open position during a normal braking event sothat both of the first and second plunger valves 2712 and 2714 are open,which may reduce noise during operation. Preferably, the first plungervalve 2712 is generally energized during an ignition cycle when theengine is running. However, the first plunger valve 2712 can purposelybe moved to its closed position in situations such as a rearwardpressure stroke of the plunger assembly 2710. The first and secondplunger valves 2712 and 2714 are preferably open when the piston of theplunger assembly 2710 is operated in its forward stroke to maximizeflow. When the driver releases the brake pedal 2808, the pressurizedfluid from the wheel brakes 16 a-d may back drive the ball screwmechanism of the plunger assembly 2710 moving the piston 2710 a back toits rest position or the plunger assembly 2710 can be moved to the restposition via driving the plunger assembly 2710 in the rearwarddirection. During this brake pedal release, the first and second plungervalves 2712 and 2714 preferably remain in their open positions. Notethat fluid can flow through the check valve within the closed secondplunger valve 2714, as well as through a check valve 2713 from thereservoir 2704 depending on the travel direction of the piston 2710 a ofthe plunger assembly 2710.

It may be desirable to configure the first plunger valve 2712 with arelatively large orifice therethrough when in its open position. Arelatively large orifice of the first plunger valve 2712 helps toprovide an easy flow path therethrough. The second plunger valve 2714may be provided with a much smaller orifice in its open position ascompared to the first plunger valve 2712. One reason for this is to helpprevent the piston 2710 a of the plunger assembly 2710 from rapidlybeing back driven upon a failed event due to the rushing of fluidthrough the first output conduit 2715 into the first pressure chamber2716 of the plunger assembly 2710, thereby preventing damage to theplunger assembly 2710. As fluid is restricted in its flow through therelatively small orifice, dissipation will occur as some of the energyis transferred into heat. Thus, the orifice should be of a sufficientlysmall size so as to help prevent a sudden catastrophic back drive of thepiston 2710 a of the plunger assembly 2710 upon failure of the brakesystem, such as for example, when power is lost to the motor and thepressure within the conduit 2717 is relatively high. Without thenormally open small orifice second plunger valve 2714, the plungerassembly 2710 moving in the rearward direction and experiencing a powerloss to the motor, the first camber 2716 would draw fluid from thereservoir 2704 via check valve 2713. However, this fluid draw from thereservoir may not be sufficient to prevent inertial forces from drivingthe plunger 2710 a into fixed parts of the plunger assembly 2710. Withthe normally open second plunger valve 2714, fluid from the secondchamber 2718 is forced through the small orifice of the second plungervalve 2714 and into the first chamber 2714. Thus, while fluid can stillbe drawn into the first chamber 2716 from the reservoir 2704, the fluidflow from the second chamber 2718 through the restricted orifice of thesecond plunger valve 2714 will act as a damping mechanism to preventinertial forces from driving the plunger 2710 a into fixed parts of theplunger assembly 2710. It is further within the scope of the inventionthat the normally open small orifice second plunger valve 2714 can alsobe used to restrict fluid flow in the event of a power loss to the motorwhen the plunger 2710 a is moving in the forward direction.

It has been found that an orifice size of about 0.85 mm diameter for theopening of the second plunger valve 2714, and an orifice size of about2.8 mm diameter for the opening of the first plunger valve 2712 issufficient. A preferred ratio for the areas of the orifices would beabout 5-20 times the area of orifice of the first plunger valve 2712compared to the area of the orifice of the second plunger valve 2714.

The first and second plunger valves 2712 and 2714 provide for an openparallel path between the pressure chambers 2716 and 2718 of the plungerassembly 2710 during a normal braking operation. Although a single openpath may be sufficient, the advantage of having both the first andsecond plunger valves 2712 and 2714 is that the first plunger valve 2712may provide for an easy flow path through the relatively large orificethereof, while the second plunger valve 2714 may provide for arestricted orifice path during certain failed conditions (when the firstplunger valve 2712 is de-energized to its closed position.

The first plunger valve 2712 may also be designed such that highpressure from the conduit 2717 assists in maintaining the first plungervalve 2712 in its closed position. Contrary, the second plunger valve2714 may be designed such that high pressure from the conduit 2717assists in maintaining the second plunger valve 2714 in its openposition. These designs may help in reducing cost of the valves 2712 and2714 so that high spring rate springs are not required and inexpensivesolenoids may be used. In addition, this functionality can also reducecurrent draw in the system.

Similar to the plunger assemblies discussed above with respect to otherbrake systems, the plunger assembly 2710 can be operated to provide asource of pressured fluid to the wheel brakes such as during a slipcontrol event. The plunger assembly 2710 can be operated in a rearwardstroke so as to provide increased pressure at the conduit 2717. In thissituation, the first and second plunger valves 2712 and 2714 arepreferably in their closed positions. Thus, the first plunger valve 2712may be de-energized, and the second plunger valve 2714 energized.

The brake system further includes solenoid actuated first and secondisolation valves 2720 and 2722, solenoid actuated apply valves 2724,2726, 2728, 2730, solenoid actuated dump valves 2732, 2734, 2736, 2738,and wheel brakes 2740 a, 2740 b, 2740 c, and 2740 d. In one embodimentof the brake system 2700, the wheel brake 2740 a is associated with theleft front wheel, the wheel brake 2740 b is associated with the rightfront wheel, the wheel brake 2740 c is associated with the left rearwheel, and the wheel brake 2740 d is associated with the right rearwheel. In this vertically split system, the first isolation valve 2720controls fluid flow to the front wheels, and the second isolation valve2722 controls fluid flow to the rear wheels. Alternatively, the brakesystem 2700 may be configured as a diagonally split system, wherein thewheel brakes 2740 b and 2740 c are associated with the front wheels, andwherein the wheel brakes 2740 a and 2740 d are associated with the rearwheels.

Referring now to the schematic representation of the brake pedal unit2702 as shown in FIG. 23, the brake pedal unit 2702 includes a housingwith a multi-stepped bore 2800 formed therein. An input piston 2802, aprimary piston 2804, and a secondary piston 2806 are slidably disposedwithin the bore 2800. The input piston 2802 is connected with a brakepedal 2808 via a linkage arm 2809. Similar in operation as the brakepedal units described above with respect to the other brake systems,leftward movement of the input piston 2802, the primary piston 2804, andthe secondary piston 2806 may cause, under certain conditions, apressure increase within the input chamber 2810, a primary chamber 2812,and a secondary chamber 2814, respectively. Various seals of the brakepedal unit 2702 as well as the structure of the housing and the pistons2802, 2804, and 2806 define the chambers 2810, 2812, and 2814. Forexample, the input chamber 2810 is generally defined between the inputpiston 2802 and the primary piston 2804. The primary chamber 2812 isgenerally defined between the primary piston 2804 and the secondarypiston 2806. The secondary chamber 2814 is generally defined between thesecondary piston 2806 and an end wall of the housing formed by the bore2800.

The input chamber 2810 is in fluid communication with the pedalsimulator via a conduit 2830. The input piston 2802 is slidably disposedin the bore 2800 of the housing of the brake pedal unit 2700. An outerwall 2832 of the input piston 2802 is engaged with a lip seal 2834 and aseal 2836 mounted in grooves formed in the housing. One or morepassageways 2838 are formed in the input piston 2802. As shown in FIGS.22 and 23, the passageway 2838 is located between the lip seal 2834 andthe seal 2836 when the input piston 2802 is in its rest position. In therest position, as shown in FIGS. 22 and 23, the passageway 2836 permitsfluid communication between the input chamber 2810 and the reservoir2704 via the open isolation valve 2758 and conduits 2756 and 2760.During activation of the power pack 2750, the isolation valve 2758 maybe energized to its closed position to prevent fluid flow from the inputchamber 2810 into the reservoir 2704 and permit fluid flow from the pumpassembly 2754. Sufficient leftward movement of the input piston 2802will cause the passageway 2838 to move past the lip seal 2834, therebypreventing the flow of fluid from the input chamber 2810 into theconduit 2756. Such sufficient leftward movement generally only happensduring an autonomous braking event such as when the power pack 2750 isoperated during a failure of the brake system 2700. Note that the lipseal 2834 is preferably configured to permit the flow of fluid in theopposite direction such that fluid may flow past the lip seal 2834 fromthe conduit 2756 into the input chamber 2810, thereby permittingpressurized fluid from the power pack assembly 2750 into the inputchamber 2810.

The primary chamber 2812 is in fluid communication with the secondisolation valve 2722 via a conduit 2840. The primary piston 2804 isslidably disposed in the bore 2800 of the housing of the brake pedalunit 2700. An outer wall 2842 of the primary piston 2804 is engaged witha lip seal 2844 and a seal 2846 mounted in grooves formed in thehousing. One or more passageways 2848 are formed in the primary piston2804. As shown in FIGS. 22 and 23, the passageway 2848 is locatedbetween the lip seal 2844 and the seal 2846 when the primary piston 2804is in its rest position. Note that in the rest position shown in FIGS.22 and 23, the lip seal 2844 is just slightly to the left of thepassageway 2848, thereby preventing fluid communication between theprimary chamber 2812 and the reservoir 2704. This prevention of fluidflow past the lip seal 2844 permits the pressure within the primarychamber 2812 to increase upon initial movement of the primary piston2804 in the leftward direction.

The secondary chamber 2814 is in fluid communication with the firstisolation valve 2720 via a conduit 2850. The secondary piston 2806 isslidably disposed in the bore 2800 of the housing of the brake pedalunit 2700. An outer wall 2852 of the secondary piston 2806 is engagedwith a lip seal 2854 and a seal 2856 mounted in grooves formed in thehousing. One or more passageways 2858 are formed in the secondary piston2806. As shown in FIGS. 22 and 23, the passageway 2858 is locatedbetween the lip seal 2854 and the seal 2856 when the secondary piston2806 is in its rest position. Note that in the rest position shown inFIGS. 22 and 23, the lip seal 2854 is just slightly to the left of thepassageway 2858, thereby preventing fluid communication between thesecondary chamber 2814 and the reservoir 2704. This prevention of fluidflow past the lip seal 2854 permits the pressure within the secondarychamber 2814 to increase upon initial movement of the secondary piston2806 in the leftward direction.

Similar to the brake pedal units described above, the primary andsecondary pistons 2804 and 2806 may be mechanically connected withlimited movement therebetween. The mechanical connection of the primaryand secondary pistons 2804 and 2806 prevents a large gap or distancebetween the primary and secondary pistons 2804 and 2806 and preventshaving to advance the primary and secondary pistons 2804 and 2806 over arelatively large distance without any increase in pressure in thenon-failed circuit. For example, if the brake system 2700 is under amanual push through mode and additionally fluid pressure is lost in theoutput circuit relative to the secondary piston 2806, such as forexample in the conduit 2850, the secondary piston 2806 will be forced orbiased in the leftward direction due to the pressure within the primarychamber 2812. If the primary and secondary pistons 2804 and 2806 werenot connected together, the secondary piston 2806 would freely travel toits further most left-hand position, as viewing FIGS. 22 and 23, and thedriver would have to depress the pedal 2808 a distance to compensate forthis loss in travel. However, because the primary and secondary pistons2804 and 2806 are connected together, the secondary piston 2806 isprevented from this movement and relatively little loss of travel occursin this type of failure. Any suitable mechanical connection between theprimary and secondary pistons 2804 and 2806 may be used. For example, asschematically shown in FIG. 23, the right-hand end of the secondarypiston 2806 includes an outwardly extending flange 2930 that extendsinto a groove 2932 formed in an inner wall 2934 of the primary piston2804. The groove 2932 has a width which is greater than the width of theflange 2930, thereby providing a relatively small amount of travelbetween the first and secondary pistons 2804 and 2806 relative to oneanother.

The brake pedal unit 2702 further includes a return spring 2870 biasingthe input piston 2802 in the rightward direction as viewing FIG. 23. Aninput spring 2872 is disposed about an axial stem 2874 formed in theinput piston 2802 and engages with a washer 2876 which is in directcontact with a shoulder 2878 formed in the right-hand end of the primarypiston 2804. The axial stem 2874 extends into a bore 2880 formed in theright-hand end of the primary piston 2804. An elastomeric pad 2882 isdisposed in the bore 2880 and will engage with an enlarged head 2883formed at the end of the axial stem 2874 when the input piston 2802 ismoved a sufficient distance towards the primary piston 2804. Compressionof the elastomeric pad 2882 by the head 2883 of the stem 2874 providesfor a desired spring rate characteristic. The enlarged head 2883 isspaced from the washer 2876 and the shoulder 2878 by a gap 2875.

The brake pedal unit 2702 further includes a primary spring 2890generally disposed between the input piston 2802 and the primary piston2804. The primary spring 2890 is disposed within the bore 2934 andengages with a retainer 2892 forming a caged spring assemblyconfiguration with an axial stem 2894 extending from bottom of the bore2934 of the primary piston 2804. The retainer 2892 is restrained by anenlarged head 2896 formed on the end of the axial stem 2894.

The brake pedal unit 2702 further includes a secondary spring 2900generally disposed between the secondary piston 2806 and a bottom wall2902 of the bore 2800. The secondary spring 2900 is disposed within abore 2904 formed in the left-hand end of the secondary piston 2806 andengages with a retainer 2908 forming a caged spring assemblyconfiguration with an axial stem 2910 extending from the bottom of thebore 2904 of the secondary piston 2806. The retainer 2908 is restrainedby an enlarged head 2912 formed on the end of the axial stem 2910. Notethat at the rest position, the enlarged head 2912 contacts the end ofthe retainer 2908 such that there is essentially no gap therebetween.

During a typical braking condition, the brake pedal 2808 is depressed bythe driver of the vehicle. In a preferred embodiment of the brake system2700, the brake pedal unit 2702 includes a pair of travel sensors 2914(for redundancy) for producing signals that are indicative of the lengthof travel of the input piston 2802 and providing the signals to anelectronic control unit (ECU) 2916. The ECU 2916 may includemicroprocessors. The ECU 2916 receives various signals, processessignals, and controls the operation of various electrical components ofthe brake system 2700 in response to the received signals. The ECU 2916can be connected to various sensors such as pressure sensors, travelsensors, switches, wheel speed sensors, and steering angle sensors. Apressure sensor 2918 detects the pressure within the secondary pressurechamber 2814 via the conduit 2850 and sends a signal indicative of thepressure to the ECU 2916. The ECU 2916 may also be connected to anexternal module (not shown) for receiving information related to yawrate, lateral acceleration, longitudinal acceleration of the vehiclesuch as for controlling the brake system 2700 during vehicle stabilityoperation. Additionally, the ECU 2916 may be connected to the instrumentcluster for collecting and supplying information related to warningindicators such as ABS warning light, brake fluid level warning light,and traction control/vehicle stability control indicator light.

During normal braking operations (normal boost apply braking operation)the plunger assembly 2710 is operated to provide boost pressure to theconduit 2717 for actuation of the wheel brakes 2740 a, 2740 b, 2740 c,2740 d, in a similar manner as described above with respect to similarbrake systems. Under certain driving conditions, the ECU 2916communicates with a powertrain control module (not shown) and otheradditional braking controllers of the vehicle to provide coordinatedbraking during advanced braking control schemes (e.g., anti-lock braking(AB), traction control (TC), vehicle stability control (VSC), andregenerative brake blending). During a normal boost apply brakingoperation, the flow of pressurized fluid from the brake pedal unit 2702generated by depression of the brake pedal 2808 is diverted into thepedal simulator 2706. The simulation valve 2708 is actuated to divertfluid through the simulation valve 2708 from the input chamber 2810.Note that fluid flow from the input chamber 2810 to the reservoir 2704is closed off once the passageway 2838 in the input piston 2802 movespast the seal 2834.

During the duration of the normal braking mode, the simulation valve2708 remains open, preferably. Also during the normal boost applybraking operation, the isolation valves 2720 and 2722 are energized tosecondary positions to prevent the flow of fluid from the conduits 2850and 2840 through the isolation valves 2720 and 2722, respectively.Preferably, the isolation valves 2720 and 2722 are energized throughoutthe duration of an ignition cycle such as when the engine is runninginstead of being energized on and off to help minimize noise. Note thatthe primary and secondary pistons 2804 and 2806 are not in fluidcommunication with the reservoir 2704 due to their passageways 2848 and2858, respectively, being positioned past the lip seals 2844 and 2854(unlike the brake pedal unit shown in FIG. 24). Prevention of fluid flowthrough the isolation valves 2720 and 2722 hydraulically locks theprimary and secondary chambers 2804 and 2814 preventing further movementof the primary and secondary pistons 2804 and 2806.

It is generally desirable to maintain the isolation valves 2720 and 2722energized during the normal braking mode to ensure venting of fluid tothe reservoir 2704 through the plunger assembly 2710 such as during arelease of the brake pedal 2808 by the driver. As shown in FIG. 22, apiston 2710 a of the plunger assembly 2710 includes a passageway 2710 bformed therein to permit this ventilation. However, during a failedcondition in which the isolation valves 2720 and 2722 are not able to beenergized, fluid from the wheel brakes 2740 a, 2740 b, 2740 c, 2740 dcan still be vented via the conduits 2840 and 2850 and through the brakepedal unit 2702 if the pressure within the primary and secondarychambers 2804 and 2806 exceeds a predetermined pressure level. In oneembodiment, the predetermined pressure level is about 0.65 bars, whichis sufficient to move the primary and secondary pistons 2804 and 2806 inthe right-hand direction such that the passageways 2848 and 2858 are tothe right of the lip seals 2844 and 2854, respectively. In thisposition, fluid can flow from the primary and secondary chambers 2812and 2814 into the reservoir 2704 via the conduits 2849 and 2859,respectively. The requirement of exceeding the predetermined pressurelevel, e.g. 0.65 bar, is generally only required during evacuation andfill bleed as well as under certain failed conditions. During a failedcondition wherein the driver is not applying pressure to the brake pedal2808, this slight pressure (about 0.65 bar for example) may cause aslight braking at the wheel brakes 2740 a, 2740 b, 2740 c, 2740 d untila volume increase in the brake fluid due to heating thereof may offsetthe pressure increase, thereby moving the primary and secondary pistons2804 and 2806 to a position to permit venting through the brake pedalunit 2702. It has been found that this minor brake application andheating will not cause undue brake fade problems at the wheel brakes.

During normal braking operations, while the pedal simulator 2706 isbeing actuated by depression of the brake pedal 2808, the plungerassembly 2710 can be actuated by the ECU 2916 to provide actuation ofthe wheel brakes 2740 a, 2740 b, 2740 c, 2740 d. The plunger assembly2710 is operated to provide desired pressure levels to the wheel brakes2740 a, 2740 b, 2740 c, 2740 d compared to the pressure generated by thebrake pedal unit 2702 by the driver depressing the brake pedal 2808.During a braking event, the ECU 2916 can also selectively actuate theapply valves 2724, 2726, 2728, 2730 and the dump valves 2732, 2734,2736, 2738 to provide a desired pressure level to the wheel brakes,respectively. The ECU 2916 can also control the brake system 2700 duringABS, DRP, TC, VSC, regenerative braking, and autonomous braking eventsby general operation of the plunger assembly 2710 in conjunction withthe apply valves 2724, 2726, 2728, 2730 and the dump valves 2732, 2734,2736, 2738.

The principle and mode of operation of this invention have beenexplained and illustrated in its preferred embodiment. However, it mustbe understood that this invention may be practiced otherwise than asspecifically explained and illustrated without departing from its spiritor scope.

What is claimed is:
 1. A brake system comprising: a plunger assemblyoperable as a pressure source to control brake fluid pressure suppliedto one or more wheel brakes, the plunger assembly including: a housinghaving first and second ports; a motor mounted on said housing fordriving an actuator; a piston connected to said actuator, said pistonslidably mounted within said housing, wherein said piston pressurizes afirst chamber when said piston is moving in a first direction to providefluid out of said first port, and wherein said piston pressurizes asecond chamber when said piston is moving in a second direction toprovide fluid out of said second port; a solenoid actuated normallyclosed first valve; and a solenoid actuated normally open second valve,wherein said first and second valves are connected in parallel betweensaid first and second ports of said plunger assembly.
 2. The brakesystem of claim 1, wherein said first valve defines a first orifice whensaid first valve is in an open position, and wherein said second valvedefines a second orifice when said second valve is in an open position,and wherein said first orifice has a greater area than said secondorifice.
 3. The brake system of claim 2, wherein said second orifice isof a sufficiently small size so as to prevent a sudden catastrophic backdrive of said piston upon failure of the brake system such that power islost to said motor and wherein said first port experiences highpressure.
 4. The brake system of claim 1 further including: a reservoir;and a check valve connected between said first port and said reservoir,wherein said check valve prevents the flow of fluid in the directionfrom said first port to said reservoir.